Include at least a syntax check into the Docker build

[Echsecutor]

It is currently possible to build a Docker image that contains broken (syntactically) python code. We should at least run pyflakes, or maybe even pep8 before shipping the image ;)

[Echsecutor]

Just an example from pyflakeing the modules which most likely hints to errors in the code

ghproj.py:11: SyntaxWarning: invalid escape sequence '\{'
  p = re.compile('domains=\{.*\}')
...
mumble.py:91:12: undefined name 'bot'
mumble.py:91:20: undefined name 'event'
mumble.py:91:30: undefined name 'bot'
mumble.py:91:43: undefined name 'event'