Resolve "Refactor Docker setup to make it more maintainable"

Closes #14 (closed)

• All hotpotato config values can now be read from the environment which will overwrite any version in a config file.
  • These are composed from HOTPOTATO_SECTION_OPTION
• The two docker images have been merged into one that slightly alters the install based on the HOTPOTATO_BUILD_DEV build arg
• dramatiq workers are no longer run while running tests
  • The whole start script is skipped so tests control database creation as well
• Having a config file is no longer required and it being missing won't throw an error
• SQLALCHEMY_TRACK_MODIFICATIONS setting has been removed from config and permanently been set to false as it doesn't expose anything useful to users and has been deprecated by flask-sqlalchemy http://flask-sqlalchemy.pocoo.org/2.1/signals/
• FLASK_DEBUG can now only be set in the environment as it is read from there directly by flask and can cause inconsistent behaviour if changed later http://flask.pocoo.org/docs/1.0/config/#environment-and-debug-features