Tag official releases

[alanbchristie]

The frontend code is cloned into the stack image as it is built but there are no tags in the frontend repo to allow older builds of the stack to be created.

Rather than use "master" as a FE_BRANCH value the stack's Dockerfile should probably use an official tag, i.e. ARG FE_GIT_PROJECT_BRANCH="1.0.0" so that unexpected frontend behaviour changes won't disrupt past or future stack builds.

Currently, if you introduce a new frontend feature/behaviour change - and you re-build an older stack - then the stack you build may contain unexpected features or behaviour.

Recommended pattern:

When a stable frontend is available...

1. Create a GitHub tag (or release) in the frontend repo (like 1.0.0)
2. Update the stack repo's Dockerfile and replace the FE_BRANCH value with tags/1.0.0