Add Docker BuildKit to CI and test pip caching

[dshemetov]

Closes #933.

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

This is a test to see if we can use Docker BuildKit in CI.

[dshemetov]

This is currently complicated by the interaction between actions/cache in CI and --mount=type=cache,target=/root/.cache/pip in Docker. It is unclear where Docker stores its cache, but it is certainly not placing those files in root/.cache/pip.

Todo:

• try using actions/cache to cache /var/lib/docker/buildkit/cache.db
• try following the instructions here

Refs:

• Someone found an approach to this, but it's not clean and uses advanced Docker features I'd rather not touch.
• This blog post explains more about how Docker mount cache works in the context of pip, but doesn't use it in a Github CI context.
• In case you need to figure out where your Python packages are stored (this is not really relevant, since we are really concerned about where Docker's cache files are, we already know that /root/.cache is the right pip artifacts directory from the blog post above).
• The RUN --mount command reference.