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Managing containers and images

Learning outcomes

After having completed this chapter you will be able to:

  • Explain the concept of layers in the context of docker containers and images
  • Use the command line to restart and re-attach to an exited container
  • Create a new image with docker commit
  • List locally available images with docker image ls
  • Run a command inside a container non-interactively
  • Use docker image inspect to get more information on an image
  • Use the command line to prune dangling images and stopped containers
  • Rename and tag a docker image
  • Push a newly created image to dockerhub
  • Use the option --mount to bind mount a host directory to a container

Material

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Exercises

Restarting an exited container

If you would like to go back to your container with the figlet installation, you could try to run again:

docker run -it ubuntu

Exercise: Run the above command. Is your figlet installation still there? Why?

Hint

Check the status of your containers:

docker container ls -a
Answer

No, the installation is gone. Another container was created from the same ubuntu image, without the figlet installation. Running the command docker container ls -a results in:

CONTAINER ID   IMAGE     COMMAND       CREATED              STATUS                     PORTS     NAMES
8d7c4c611b70   ubuntu    "/bin/bash"   About a minute ago   Up About a minute                    kind_mendel
27f7d11608de   ubuntu    "/bin/bash"   27 minutes ago       Exited (0) 2 minutes ago             great_moser

In this case the container great_moser contains the figlet installation. But we have exited that container. We created a new container (kind_mendel in this case) with a fresh environment created from the original ubuntu image.

To restart your first created container, you’ll have to look up its name. You can find it in the Docker dashboard, or with docker container ls -a.

Container names

The container name is the funny combination of two words separated by _, e.g.: nifty_sinoussi. Alternatively you can use the container ID (the first column of the output of docker container ls)

To restart a container you can use:

docker start [CONTAINER NAME]

And after that to re-attach to the shell:

docker attach [CONTAINER NAME]

And you’re back in the container shell.

Exercise: Run the docker start and docker attach commands for the container that is supposed to contain the figlet installation. Is the installation of figlet still there?

Answer

yes:

figlet 'try some more text!'

Should give you output.

docker attach and docker exec

In addition to docker attach, you can also “re-attach” a container with docker exec. However, these two are quite different. While docker attach gets you back to your stopped shell process, docker exec creates a new one (more information on stackoverflow). The command docker exec enables you therefore to have multiple shells open in the same container. That can be convenient if you have one shell open with a program running in the foreground, and another one for e.g. monitoring. An example for using docker exec on a running container:

docker exec -it [CONTAINER NAME] /bin/bash

Note that docker exec requires a CMD, it doesn’t use the default.

Creating a new image

You can store your changes and create a new image based on the ubuntu image like this:

docker commit [CONTAINER NAME] ubuntu-figlet

Exercise: Run the above command with the name of the container containing the figlet installation. Check out docker image ls. What have we just created?

Answer

A new image called ubuntu-figlet based on the status of the container. The output of docker image ls should look like:

REPOSITORY                        TAG       IMAGE ID       CREATED         SIZE
ubuntu-figlet                     latest    e08b999c7978   4 seconds ago   101MB
ubuntu                            latest    f63181f19b2f   29 hours ago    72.9MB

Now you can generate a new container based on the new image:

docker run -it ubuntu-figlet

Exercise: Run the above command. Is the figlet installation in the created container?

Answer

yes

Commands

The second positional argument of docker run can be a command followed by its arguments. So, we could run a container non-interactively (without -it), and just let it run a single command:

docker run ubuntu-figlet figlet 'non-interactive run'

Resulting in just the output of the figlet command.

In the previous exercises we have run containers without a command as positional argument. This doesn’t mean that no command has been run, because the container would do nothing without a command. The default command is stored in the image, and you can find it by docker image inspect [IMAGE NAME].

Exercise: What is the default command (CMD) of the ubuntu image?

Answer

Running docker image inspect ubuntu gives (amongst other information): 

"Cmd": [
    "bash"
],
In the case of the ubuntu the default command is bash, returning a shell in bash (i.e. Bourne again shell). Adding the options -i and -t (-it) to your docker run command will therefore result in an interactive bash shell. You can modify this default behaviour. More on that later, when we will work on Dockerfiles.

Removing containers

In the meantime, with every call of docker run we have created a new container (check your containers with docker container ls -a). You probably don’t want to remove those one-by-one. These two commands are very useful to clean up your Docker cache:

  • docker container prune: removes stopped containers
  • docker image prune: removes dangling images (i.e. images without a name)

So, remove your stopped containers with:

docker container prune

Unless you’re developing further on a container, or you’re using it for an analysis, you probably want to get rid of it once you have exited the container. You can do this with adding --rm to your docker run command, e.g.:

docker run --rm ubuntu-figlet figlet 'non-interactive run'

Pushing to dockerhub

Now that we have created our first own docker image, we can store it and share it with the world on docker hub. Before we get there, we first have to (re)name and tag it.

Before pushing an image to dockerhub, docker has to know to which user and which repository the image should be added. That information should be in the name of the image, like this: user/imagename. We can rename an image with docker tag (which is a bit of misleading name for the command). So we could push to dockerhub like this:

docker tag ubuntu-figlet [USER NAME]/ubuntu-figlet
docker push [USER NAME]/ubuntu-figlet

If on Linux

If you are on Linux and haven’t connected to docker hub before, you will have login first. To do that, run:

docker login

How docker makes money

All images pushed to dockerhub are open to the world. With a free account you can have one image on dockerhub that is private. Paid accounts can have more private images, and are therefore popular for commercial organisations. As an alternative to dockerhub, you can store images locally with docker save.

We didn’t specify the tag for our new image. That’s why docker tag gave it the default tag called latest. Pushing an image without a tag will overwrite the current image with the tag latest (more on (not) using latest here). If you want to maintain multiple versions of your image, you will have to add a tag, and push the image with that tag to dockerhub:

docker tag ubuntu-figlet [USER NAME]/ubuntu-figlet:v1
docker push [USER NAME]/ubuntu-figlet:v1

Mounting a directory

For many analyses you do calculations with files or scripts that are on your host (local) computer. But how do you make them available to a docker container? You can do that in several ways, but here we will use bind-mount. You can bind-mount a directory with -v (--volume) or --mount. Most old-school docker users will use -v, but --mount syntax is easier to understand and now recommended, so we will use the latter here:

docker run \
--mount type=bind,source=/host/source/path,target=/path/in/container \
[IMAGE]

The target directory will be created if it does not yet exist. The source directory should exist.

MobaXterm users

You can specify your local path with the Windows syntax (e.g. C:\Users\myusername). However, you will have to use forward slashes (/) instead of backward slashes (\). Therefore, mounting a directory would look like:

docker run \
--mount type=bind,source=C:/Users/myusername,target=/path/in/container \
[IMAGE]

Do not use autocompletion or variable substitution (e.g. $PWD) in MobaXterm, since these point to ‘emulated’ paths, and are not passed properly to the docker command.

Using docker from Windows PowerShell

Most of the syntax for docker is the same for both PowerShell and UNIX-based systems. However, there are some differences, e.g. in Windows, directories in file paths are separated by \ instead of /. Also, line breaks are not escaped by \ but by `.

Exercise: Mount a host (local) directory to a target directory /working_dir in a container created from the ubuntu-figlet image and run it interactively. Check whether the target directory has been created.

Answer

e.g. on Mac OS this would be:

docker run \
-it \
--mount type=bind,source=/Users/myusername/working_dir,target=/working_dir/ \
ubuntu-figlet

This creates a directory called working_dir in the root directory (/):

root@8d80a8698865:/# ls
bin   dev  home  lib32  libx32  mnt  proc  run   srv  tmp  var
boot  etc  lib   lib64  media   opt  root  sbin  sys  usr  working_dir

This mounted directory is both available for the host (locally) and for the container. You can therefore e.g. copy files in there, and write output generated by the container.

Exercise: Write the output of figlet "testing mounted dir" to a file in /working_dir. Check whether it is available on the host (locally) in the source directory.

Hint

You can write the output of figlet to a file like this: 

figlet 'some string' > file.txt

Answer 
root@8d80a8698865:/# figlet 'testing mounted dir' > /working_dir/figlet_output.txt

This should create a file in both your host (local) source directory and the target directory in the container called figlet_output.txt.

Using files on the host

This of course also works the other way around. If you would have a file on the host with e.g. a text, you can copy it into your mounted directory, and it will be available to the container.

Managing permissions (extra)

Depending on your system, the user ID and group ID will be taken over from the user inside the container. If the user inside the container is root, this will be root. That’s a bit inconvenient if you just want to run the container as a regular user (for example in certain circumstances your container could write in /). To do that, use the -u option, and specify the group ID and user ID like this:

docker run -u [uid]:[gid]

So, e.g.:

docker run \
-it \
-u 1000:1000 \
--mount type=bind,source=/Users/myusername/working_dir,target=/working_dir/ \
ubuntu-figlet

If you want docker to take over your current uid and gid, you can use:

docker run -u "$(id -u):$(id -g)"

This behaviour is different on MacOS and MobaXterm

On MacOS and in the local shell of MobaXterm the uid and gid are taken over from the user running the container (even if you set -u as 0:0), i.e. your current ID. More info on stackoverflow.

Exercise: Start an interactive container based on the ubuntu-figlet image, bind-mount a local directory and take over your current uid and gid. Write the output of a figlet command to a file in the mounted directory. Who and which group owns the file inside the container? And outside the container? Answer the same question but now run the container without setting -u.

Answer

Running ubuntu-figlet interactively while taking over uid and gid and mounting my current directory:

docker run -it --mount type=bind,source=$PWD,target=/data -u "$(id -u):$(id -g)" ubuntu-figlet
Inside container:

I have no name!@e808d7c36e7c:/$ id
uid=1000 gid=1000 groups=1000

So, I have taken over uid 1000 and gid 1000.

I have no name!@e808d7c36e7c:/$ cd /data
I have no name!@e808d7c36e7c:/data$ figlet 'uid set' > uid_set.txt
I have no name!@e808d7c36e7c:/data$ ls -lh
-rw-r--r-- 1 1000 1000 0 Mar  400 13:37 uid_set.txt

So the file belongs to user 1000, and group 1000.

Outside container:

ubuntu@ip-172-31-33-21:~$ ls -lh
-rw-r--r-- 1 ubuntu ubuntu 400 Mar  5 13:37 uid_set.txt

Which makes sense:

ubuntu@ip-172-31-33-21:~$ id
uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu)

Running ubuntu-figlet interactively without taking over uid and gid:

docker run -it --mount type=bind,source=$PWD,target=/data ubuntu-figlet
Inside container:

root@fface8afb220:/# id
uid=0(root) gid=0(root) groups=0(root)

So, uid and gid are root.

root@fface8afb220:/# cd /data
root@fface8afb220:/data# figlet 'uid unset' > uid_unset.txt
root@fface8afb220:/data# ls -lh
-rw-r--r-- 1 1000 1000 400 Mar  5 13:37 uid_set.txt
-rw-r--r-- 1 root root 400 Mar  5 13:40 uid_unset.txt

Outside container:

ubuntu@ip-172-31-33-21:~$ ls -lh
-rw-r--r-- 1 ubuntu ubuntu 0 Mar  5 13:37 uid_set.txt
-rw-r--r-- 1 root   root   0 Mar  5 13:40 uid_unset.txt

So, the uid and gid 0 (root:root) are taken over.

Running ubuntu-figlet interactively while taking over uid and gid and mounting my current directory:

docker run -it --mount type=bind,source=$PWD,target=/data -u "$(id -u):$(id -g)" ubuntu-figlet
Inside container:

I have no name!@e808d7c36e7c:/$ id
uid=503 gid=20(dialout) groups=20(dialout)
So, the container has taken over uid 503 and group 20

I have no name!@e808d7c36e7c:/$ cd /data
I have no name!@e808d7c36e7c:/data$ figlet 'uid set' > uid_set.txt
I have no name!@e808d7c36e7c:/data$ ls -lh
-rw-r--r--  1 503 dialout    400 Mar  5 13:11 uid_set.txt

So the file belongs to user 503, and the group dialout.

Outside container:

mac-34392:~ geertvangeest$ ls -lh
-rw-r--r--   1 geertvangeest  staff     400B Mar  5 14:11 uid_set.txt

Which are the same as inside the container:

mac-34392:~ geertvangeest$ echo "$(id -u):$(id -g)"
503:20

The uid 503 was nameless in the docker container. However the group 20 already existed in the ubuntu container, and was named dialout.

Running ubuntu-figlet interactively without taking over uid and gid:

docker run -it --mount type=bind,source=$PWD,target=/data ubuntu-figlet
Inside container:

root@fface8afb220:/# id
uid=0(root) gid=0(root) groups=0(root)

So, inside the container I am root. Creating new files will lead to ownership of root inside the container:

root@fface8afb220:/# cd /data
root@fface8afb220:/data# figlet 'uid unset' > uid_unset.txt
root@fface8afb220:/data# ls -lh
-rw-r--r--  1 503 dialout    400 Mar  5 13:11 uid_set.txt
-rw-r--r--  1 root root    400 Mar  5 13:25 uid_unset.txt

Outside container:

mac-34392:~ geertvangeest$ ls -lh
-rw-r--r--   1 geertvangeest  staff     400B Mar  5 14:11 uid_set.txt
-rw-r--r--   1 geertvangeest  staff     400B Mar  5 14:15 uid_unset.txt

So, the uid and gid 0 (root:root) are not taken over. Instead, the uid and gid of the user running docker were used.

Running ubuntu-figlet interactively while taking over uid and gid and mounting to a specfied directory:

docker run -it --mount type=bind,source=C:/Users/geert/data,target=/data -u "$(id -u):$(id -g)" ubuntu-figlet
Inside container:

I have no name!@e808d7c36e7c:/$ id
uid=1003 gid=513 groups=513
So, the container has taken over uid 1003 and group 513

I have no name!@e808d7c36e7c:/$ cd /data
I have no name!@e808d7c36e7c:/data$ figlet 'uid set' > uid_set.txt
I have no name!@e808d7c36e7c:/data$ ls -lh
-rw-r--r--  1 1003 513    400 Mar  5 13:11 uid_set.txt

So the file belongs to user 1003, and the group 513.

Outside container:

/home/mobaxterm/data$ ls -lh
-rwx------   1 geert  UserGrp     400 Mar  5 14:11 uid_set.txt

Which are the same as inside the container:

/home/mobaxterm/data$ echo "$(id -u):$(id -g)"
1003:513

Running ubuntu-figlet interactively without taking over uid and gid:

docker run -it --mount type=bind,source=C:/Users/geert/data,target=/data ubuntu-figlet
Inside container:

root@fface8afb220:/# id
uid=0(root) gid=0(root) groups=0(root)

So, inside the container I am root. Creating new files will lead to ownership of root inside the container:

root@fface8afb220:/# cd /data
root@fface8afb220:/data# figlet 'uid unset' > uid_unset.txt
root@fface8afb220:/data# ls -lh
-rw-r--r--  1 1003 503    400 Mar  5 13:11 uid_set.txt
-rw-r--r--  1 root root    400 Mar  5 13:25 uid_unset.txt

Outside container:

/home/mobaxterm/data$ ls -lh
-rwx------   1 geert  UserGrp     400 Mar  5 14:11 uid_set.txt
-rwx------   1 geert  UserGrp     400 Mar  5 14:15 uid_unset.txt

So, the uid and gid 0 (root:root) are not taken over. Instead, the uid and gid of the user running docker were used.

Questions & Answers