add dockerfile to repo to aid in development work

[andy-esch]

Ideally the docker image would be:

• default configuration is to run tests
• configurable for different versions of PostgreSQL (9.5, 9.6, 10) and Ubuntu (precise, xenial)
• include instructions on having the crankshaft repo in a shared directory so development can happen outside of the container but be easily installed in the container

There is a dockerfile in this fork of crankshaft that is just ported from the travis.yml