Document how to trigger Docker image builds for specific branches

Created by: mrnugget

I was looking for instructions on how to do this again today but couldn't find anything, so here's my attempt to add them.

Problem: I don't know if they still work. My build failed https://buildkite.com/sourcegraph/sourcegraph/builds/56652#05c7ad40-57a0-4daa-9352-3634331adf35 but pushed the image up.

Tagging @ggilmore and @keegancsmith here because you two might know what changed.