Minor updates to postgresql_service.rst

I've added the steps that were missing/wrong for me when I went through this just now.
tell the new developer how they can replace the real docker binary with the one they've been hacking on (#2142)
Merge pull request #2142 from SvenDowideit/patch-1

tell the new developer how they can replace the real docker binary with ...
Merge pull request #2160 from iffy/patch-1

Minor updates to postgresql_service.rst