Can't get the container to shut down cleanly.

During testing, I've run into this annoying problem frequently:

2014-05-25 20:08:02 UTC FATAL:  lock file "postmaster.pid" already exists
2014-05-25 20:08:02 UTC HINT:  Is another postmaster (PID 14) running in data directory "/data"?

The process structure looks like this:

      647     1 docker           /usr/local/bin/docker -d -D -g /var/lib/docker -H unix:// -H tcp://0.0.0.0:4243
    16329   647 start-flynn-pos  {start-flynn-pos} /bin/bash /bin/start-flynn-postgres postgres
    16339 16329 sudo             sudo -u postgres -H EXTERNAL_IP=10.0.2.15 PORT=5555 DISCOVERD=192.168.59.103:1111 /bin/flynn-postgres
    16340 16339 flynn-postgres   /bin/flynn-postgres
    16346 16340 postgres         /usr/lib/postgresql/9.3/bin/postgres -D /data -p 5555 -h * -l

Starting with 16339, I can do ``kill -s SIGTERM`` and have the container shutdown. Sending SIGTERM to 16329 has no effect - which is why docker stop will do a SIGKILL.

I don't understand what the strange ``{start-flynn-postgres}``syntax means, but I wonder if the reason is the sudo call and the permission boundaries.

It does work if ``exec`` is used, so this is what the pull request does.