Docker: producing the official Heptapod image directly
Since we don't plan to support Python 2 any more (heptapod#352 (closed)), with Omnibus now providing the inner HTTP service (!5 (merged)), we can simply use the Docker image provided by Omnibus.
There was a catch in the startup sequence, with a check that has been added in GitLab since we copied it over to Heptapod Docker.