Drop EXPOSE line from Dockerfile

It's only for documentation, the `app_local_port` may not correspond to
the port expected in production, etc.

Author: doshitan

template/{{app_name}}/Dockerfile.jinja renamed to template/{{app_name}}/Dockerfile

@@ -19,9 +19,6 @@ ENV RAILS_ENV="production" \
     BUNDLE_PATH="/usr/local/bundle" \
     TMPDIR="/rails/tmp"
 
-# Start the server by default, this can be overwritten at runtime
-EXPOSE {{ app_local_port }}
-
 # In production, this should be the only filesystem location with writes
 VOLUME /rails/tmp