Add workflow to tag latest docker image on release

[drogus]

Description of Changes

This PR adds a workflow to tag a docker image linked to the release as latest

Expected complexity level and risk

Low, just a workflow change

[kurtismullins]

@drogus -- Looks great from simply reading it but it will be impractical to test until our next release.

We should confirm with @cloutiertyler and @jdetter that we want to auto-tag spacetimedb:latest because I vaguely remember someone mentioning they want it to be a manual step. I could be wrong!

[drogus]

Looks great from simply reading it but it will be impractical to test until our next release.

Yeah, that's the biggest problem I think 😅 fwiw I've tested it on a test repo and it worked correctly - the only thing that I chnaged bringing it back here is the docker image name

We should confirm with @cloutiertyler and @jdetter that we want to auto-tag spacetimedb:latest because I vaguely remember someone mentioning they want it to be a manual step. I could be wrong!

👍

[kurtismullins]

I saw the verbal approval from John that this is the release behavior we want. Ship it!