Improve action's setup time

[Jackenmen]

Currently, most of the time is spent on the setup of the action - especially the installation of the Python package. It would be good to look into ways of improving that such as:

• caching dependencies and the installed package (perhaps whole virtualenv) based on the action commit
• building a wheel and storing it as a release artifact (would probably require additional fetching unless I put it as part of the repository)
• switching from composite action to Docker action
  • this might not be the optimal direction as Docker actions are likely to be slower to set up (but the docker image is surely cached)

[Jackenmen]

Something worth noting:

Docker actions support pulling in pre-built Docker images by supplying a registry address to the image: key. The downside to this syntax is that there's no way to specify the correct Docker tag because the GitHub Actions image: and uses: keys don't accept any context. For example, if a user's workflow has uses: pypa/gh-action-pypi-publish@release/v1.8, then the action should pull in a Docker image built from the release/v1.8 ref, something like ghcr.io/pypa/gh-action-pypi-publish:release-v1.8 (Docker tags can't have /).

# this works but the image tag can't be customized
runs:
  using: docker
  image: docker://ghcr.io/pypa/gh-action-pypi-publish:release-v1.8

# this doesn't work because `image:` doesn't support context
runs:
  using: docker
  image: docker://ghcr.io/pypa/gh-action-pypi-publish:${{ github.action_ref }}

The workaround is to switch the top-level action.yml to a composite action that then calls the Docker action, substituting the correct image name and tag.

Source: pypa/gh-action-pypi-publish#230