build-unit-test-docker.sh: Enable branch support

[geissonator]

When we branch for our first release next February, we'll need this script to support taking a branch as input. Can't just keep building the latest master of the dependent repos when a commit is going into a specific branch.

I'm thinking a 3rd parameter where a branch name could be passed in. The script would then retrieve the latest commit in that branch from each dependent repo instead of head.

Thoughts/Comments @amboar @msbarth @wak-google @pstrinkle ?