improve image building approach

[mathematicalmichael]

• moved image config to build directory
• added github action to publish an image to github packages
• image can be pulled by binder or the user without having to build it, should speed things up

TODO:

• add root Dockerfile to reference the built image
• test that binder still works, and doesn't take nearly as long as before
• test docker-compose instructions
• add metadata: https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry

[mathematicalmichael]

apparently I can't use latest as a tag in my binder Dockerfile, must specify a SHA (hash) explicitly.
this makes me think that release -> triggers a commit to the same branch that updates the root Dockerfile.

For testing purposes, I'll handle this step manually for the time being as this "get github actions to add a commit" step seems potentially complicated.