docker: better boot procedure

• BETTER Integrates docker boot script into docker image.

• Downgrades our "only for develepers" warnings to "please adjust before use". Actually the docker image security is pretty good and only the DB+cache connection details might need some adjustment for production use.

• BETTER Changes docker boot script to use exec. This ensure signal forwarding and reduces the overhead by one process. As a result container shutdown is faster now.

Signed-off-by: Marco Neumann <marco@crepererum.net>