docker: Initialise postfix correctly

Postfix initialisation is a bit more complicated than the service file assumed
and doesn't actually work due to pieces missing from the chroot. Delegate to
the sysv init script and tail the log to keep the process running.

This is potentially a problem if postfix crashes and doesn't come back, but I
reckon finding a crashing bug in postfix is a few orders of magnitude less
likely than finding a similar bug in CKAN.