ci: test the Dockerfile
ci: test the Dockerfile

Add a `Test Dockerfile` job to Travis that checks the `docker build` exit
code.
More useful tests can be added later.

joequant/etherpad-lite,aptivate/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite