build-unit-test-docker: create hashed tags for final image

Create a tag for the final image in the same format as the stage
images.  Display this tag to stdout at the end of the docker build
so that other scripts can consume it via calls like:
    $(scripts/build-unit-test-docker)

Modify `run-unit-test-docker.sh` to get the docker image from
`build-unit-test-docker` rather than a static image name.  This
ensures that the docker image used in unit testing is the exact
image requested to be built.  (Soon this will allow us to incorporate
Gerrit changes into the docker image itself.)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Idd7a9a7b0157f1c520b33b3ec7ac0d5b52018d9e
2 files changed