Add documentation on using Singularity

[enasca]

In preparation of switching from Docker to Singularity Apptainer, I'd like to create a document containing a few Docker use cases and their Singularity equivalent. All of the listed examples should be things that we've tried on the cluster.

• [ ] Becoming a regular user.
  • [ ] Are files created in the container owned by you on the host?
  • [ ] Does the user retain its username, UID and primary GID in the container?
  • [ ] Is it possible to bind-mount project directories at any level (e.g. /projects/myproject/mydir)?
• [ ] Becoming root.
  • [ ] Are files created in the container owned by you on the host?
  • [ ] Can you do things such as installing packages in the container?
• [ ] Launching a server application
  • [ ] How to launch it with the default parameters?
  • [ ] How to map the container port to a different host port?
• [ ] Putting two or more containers in communication
  • [ ] Do we need to expose a port on the host? For example, by launching a server with docker network create mynetwork docker run --name myserver --network mynetwork nginx a client container can contact it by joining the same network, with no ports exposed on the host: docker run --name myclient --network mynetwork curlimages/curl http://myserver
• [ ] Using GPUs
  • [ ] If we request one or more GPUs via slurm, do we get them in Singularity too?
• [ ] Dockerfiles
  • [ ] What if users want to build local Dockerfiles? Maybe we can provide a registry so that people can build+push from their own machine and then use the image with Apptainer.
• [ ] Docker Compose files
  • [ ] Perhaps we could offer https://github.com/containers/podman-compose alongside Apptainer. I haven't seen an Apptainer implementation

[enasca]

It's actually apptainer now.

[enasca]

Added a checkbox about Docker Compose.

[enasca]

Added a checkbox about networking between containers.

[enasca]

Added a checkbox about local Dockerfiles.

[enasca]

These are good starting points for people already familiar with Docker:

• https://apptainer.org/docs/user/main/quick_start.html
• https://apptainer.org/docs/user/main/docker_and_oci.html#differences-and-limitations-vs-docker

It's important that we recommend to put sandbox directories on local storage, i.e. in /local/work.