Run mailman inside the container as a non-root process. (#31)

* Run as mailman user.

* Run mailman core process as a less-privileged mailman user.

* Add a `docker ps` to travis to know if any container died.

* Spit out more debug information in CI.

* Install su-exec properly.

core/assets/run.sh

@@ -122,4 +122,7 @@ base_url: $HYPERKITTY_URL
 api_key: $HYPERKITTY_API_KEY
 EOF
 
-exec "$@"
+# Now chown the places where mailman wants to write stuff.
+chown -R mailman /opt/mailman
+
+exec su-exec mailman "$@"