automatic generation of latest tag fails when buildstream changes

For some reason, the docker build is tied into the buildstream repository itself, generating it's tags based on whatever is master in buildstream at the time, which tends to change in between a docker build and a docker push CI task.

To reproduce:

• Start a docker build task
• Push a commit to buildstream master

The docker "push" task will then assume a different intermediate tag to publish as latest resulting in a "no such tag" error.