Up to this point, the project has been set up as a Docker action
referencing the Dockerfile. The downside to using the Dockerfile for the
action is that the Docker image must be built every time the action is
used.

This commit will set up the project to build the Docker image and push
it to GitHub Container Registry (GHCR). This change will speed up user
workflows every time the action is used because the workflows will
simply pull the Docker image from GHCR instead of building again.

Changes:

- Add required metadata to Dockerfile
- Build container image with GitHub Actions
- Push container image to GHCR

Docker actions support pulling in pre-built Docker images. The downside
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` branch, something
like `ghcr.io/pypa/gh-action-pypi-publish:release-v1.8` (Docker tags
can't have `/`). 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.

pypa#230 (comment)