Fix latest tag in Docker image workflows

[nguyer]

Right now our docker image GitHub workflows have a problem where if we release a 1.2.x patch release after a 1.3.0, the 1.2.x release will get tagged as latest which will potentially "downgrade" people if they try to pull latest. I think most people would expect 1.3.0 to remain the latest in this case.

I wrote a new way of determining whether a workflow is running as the result of being the latest release, or just some other release and it's part of the docs site build here: https://github.com/hyperledger/firefly/blob/main/.github/workflows/docs.yml#L26-L48

We should port this code into the docker release job as well: https://github.com/hyperledger/firefly/blob/main/.github/workflows/docker_release.yml

This definitely needs to happen before we do a 1.2.3 patch release or we will need to manually fix Docker tags as we have in the past.