enable different container technologies (start with apptainer)

If we want to enable containerization on HPC hardware we need to add support for their container technologies. I would start with Apptainer (Singularity) which to my knowledge is the most widely used one but we should keep our options open and make this adaptable to different technologies.

I believe the actual porting of the containers won't be such a big issue since most of them support conversion out of the box (I think it is only fair to support those in the beginning). I tried pulling the stdlib container from dockerhub and was sent to the python environment so everything seems to work as expected on that side.

For Port, I don't fully grasp which functionality (we don't have any special network things which might not work the same in unpriviledged container runtimes like apptainer, the GPU support comes from docker but has equivalents in apptainer) is dependent on docker. It might be a good idea to have ContainerRuntime (which is not the WhaleRuntime, or is it?), which changes depending on the technology used.

No files, tests, or concrete entry points are named. Start by locating the Port, WhaleRuntime, and stdlib container references, then determine which Docker-dependent functionality must be abstracted for Apptainer; done means container support works on HPC hardware and the runtime remains adaptable to other technologies.

Written by the indexing model from the issue text.