docker: Set up for building our image and running tests/code

[gliga]

The goal of this PR is to enable us to run pykokkos in a docker container. The items included:

• Dockerfile to let us build the image (this is focusing on CPU runs at the moment)
• Script that can use pykkokos image to run tests and examples in a container
• requirements.txt to ensure more control over versioning

Some decision decisions:

• Use pykokkos as the name for the image (note that later we might add other images for -cuda or -hip). Maybe -cpu could be added, but I did not feel like it.
• Use pk as the username (because I felt there are many pykokkos things around already).
• Potentially went a bit too far with being strict on version numbers, but felt good about it (as we can relax things in the future)

One thing we will probably want to add into the pk script is to pull the the pykokkos image if one is not present.