Use runit to start/manage the process in the container

This change allows chained commands for an entry in the Procfile since the startup command is
a simple shell script (Fixes #114)

All entries in a Procfile get converted into service definitions on build, the /start command
determines on container start which one is to be started, ignoring all others.

If no Procfile ist found the default process types from the .release file will get used.
Propagate TERM signal to all child processes

Using the finish script called by runit after the main process has
ended determine the process group id and send the TERM signal to all
children. (Fixes #102)