greater capacity: 64 avatars and docker-engine

cyber-dojo now supports a docker-engine runner and http://cyber-dojo.org is using it to run a small cluster of Google-compute nodes behind the server. In line with this extra capacity each cyber-dojo can now have 64 avatars. A big thank you to everyone who helped with this work, particularly Byran Wills Heath from Bluefruit Software who refactored the containers, Mike Long for helping with the architecture design, and Dymtro Mindra and Nadya Sivers for providing all the new avatar images.