Readthedocs failed to render Drydock exceptions with error:
> WARNING: autodoc: failed to import exception xxx from module
> 'drydock_provisioner'; the following exception was raised: No module
> named 'drydock_provisioner'
Trying to add Drydock requirements to the installed requirements list,
so that Readthedocs has all modules, including those needed for the
Unify docs building by utilizing Zuul docs-on-readthedocs template job.
Cosmetic readability changes:
1. combined all Makefile .PHONY targets into one
2. merged multiple LABEL instructions in Dockerfile into one