Add script to run at container start

Added script to perform actions at container start (possibly based on
environment variables). Moved ENTRYPOINT from Dockerfile to the end of
this script and replaced it with CMD to run the script.
No longer build dovecot from mail-base image

The alias, domain, and password files are no longer build into the
image. They must now be provided to the container in the directory
/mail-base. This can be achieved either by mounting a directory on the
host or by using the --volumes-from option with the modified mail-base
image.
Add fig file for easying container management