Release procedure

A reminder for the maintainers on how to make a new release. Releases are made by creating a Git tag with a semantic version and pushing to GitHub.

Make sure all your changes are committed (including an entry in CHANGELOG.rst). Then run:

$ git tag -s X.Y.Z -m "X.Y.Z"
$ git push
$ git push --tags

A GitHub action will build the new version and upload to Docker Hub.