Docker tips - for rootless Docker¶

Rootless Docker is a way of running the Docker daemon in the users own namespace, and does not require the user to be in a special group, or to have sudo permissions. However, it has limitations (see above) and requires an initial setup. On the CS machines, for the moment, the admin will need to setup each user to be able to use rootless Docker. This will be automated in future.

Initialize¶

1. Test that you are present in the /etc/subguid and /etc/subuid files. There will be output containing your username:

   grep ^$(whoami): /etc/subuid
   grep ^$(whoami): /etc/subgid
   

2. If you are not in that file, you will need to contact the admin for help. If you are then run:

   dockerd-rootless-setuptool.sh install
   

   • Read the output carefully. If successful, follow the instructions about environment variables you should set in your .bashrc file.
   • If not, then there should be some output that you can send to the system admin.

3. If everything went well, run the following to make sure your Docker daemon stays alive, and is started on boot:

   systemctl --now --user enable docker
   loginctl enable-linger $(whoami)
   

4. Finally, test that you can use Docker:

   docker run hello-world
   docker ps
   docker images ls
   

Running containers¶

If you know that you do not need the docker container once its stopped, you can use the --rm flag when executing docker run .... This will automatically remove the container from the host once it has stopped.

It's probably good practice to use this, since it forces a user to make sure any important info and data is stored in a volume.

The biggest drawback here is that you will not be able to read the container log once the container stops, since it will have been removed.

Naming¶

Images¶

If an image has been pulled from a remote registry, it will already have a name. This is OK, since you can just pull that image again if it's removed.

However, if the image was built locally (from a Dockerfile for example) the image name will need to be set at build time.

This can be done on the command line³:

docker build --tag <name[:tag]> <context>

For example:

docker build --tag ajcollett:v2.1 .

Containers¶

There are a few ways to name containers, however we will only cover the simplest here.

In order to add a name to a container when you run it use the following docker run option⁵:

docker run --name <your username + extra helpful things> <image name>

For example:

docker run --name ajcollett_pythonapp_v1 -it python:latest

Cleaning up¶

It is helpful for users to cleanup old containers and images they may have created, if they have not used --rm when running containers.

W.I.P