Consolidate docker builds on `unstable`, `stable` and tag builds during release

[jimmygchen]

Description

We currently trigger three separate docker workflows during a release:

1. stable branch: this builds and publishes the latest images
2. unstable branch: this builds and publishes the -unstable images
3. tag: this builds and publishes the version tagged images

and each of these trigger 9 build jobs for different platform / archs, so:

• the build queue could get quite long, and this prolongs the release process
• the images would have different digests even though they're built from the exact same source, which could be a bit confusing
• it is quite wasteful in terms of compute resources

These three workflows are built from the exact same commit, so we could probably consolidate them, so we only build each image once, and tag the same image with latest, -unstable and the version.