We have a Dockerfile floating around in dev-support. It looks like it's only used by the flaky-test Jenkins job, so move it under that directory. I think that it used to be used in multiple places, as way to package up the dev-support/python-requirements.txt file. However, that too seems to only be used by the flaky-tests job, so move it as well. Update the job for the new invocation form, and remove mentions of python-requirements.txt from the book – an old, out-dated section about how to format patches for attachment to JIRA.