WIP: Build a docker image for every branch

What does this Merge Request do?

Creates a new docker image for every merge request instead of testing against the latest image. Publishes the latest image from master after a successful pipeline.

There are a couple of benefits to this approach:

• Changes within the dockerfile don't have to be published to the world as latest before being tested.
• The latest image is guaranteed to originate from master (and pipeline is guaranteed to have been successful).

There is a downside however:

• The multi-stage caching doesn't quite work correctly yet, and building an image takes ~15 minutes.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.

Closes #755 (closed)