Fix broken Docker build

See #1312.

It seems like this new Dockerfile is building correctly. As evidence, we can see a new Docker hub trusted build I created for this branch: [https://registry.hub.docker.com/u/allanino/new-nupic/builds_history/55181/](https://registry.hub.docker.com/u/allanino/new-nupic/builds_history/55181/).