Implement better way to release new version

[crozzy]

Currently the CI to cut a new Release takes a tag and checks that the Dockerfile has been updated with the new tag. If it hasn't, the release process fails. This is a weird chicken and egg situation where you need to prepare the code for a tag before it exists, but doing it after doesn't work either.

This probably merits a prepare-release workflow that creates the PR iterating the image version in the Dockerfile that needs merging before cutting a tag. It doesn't totally solve the situation but at least gives the operator a better defined procedure.