Only build environment image per commit hash

[justingrayston]

Currently the Cloud Build configs built the environment every run which is wasteful as the code underneath hasn't actually changed necessarily (for example promoting to higher environments).

Why build anyway?

The pip install from requirements could take up to 8 minutes to run as the grpc dependency takes a while to load (especially on a Cloud Build machine). It is much quicker to build the docker image than it to run it on the skinny build image as more dependencies are already present.

This is obviously subject to change and future testing is needed.