Dockerize unit tests

[ahouseholder]

This PR adds a Dockerfile that installs the requirements and runs the included unit tests.

Usage:

$ docker build -t ssvc_test .
...
$ docker run -it --rm ssvc_test 
............................................................ [100%]
60 passed in 0.71s

I'd like to eventually dockerize the dev server too, but that will come later.

[sei-vsarvepalli]

Docker works well - useful.