add Docker support

[wesley-dean]

To simplify distribution and usage of the fcli tool, please consider adding Docker scaffolding to build an image that can be run independent of installed Python versions, package versions, etc. on the host system.

This way, to use the tool, one would run the appropriate image such that fcli executed in the context of the container like so:

docker run -v ~/.fcli:/.fcli halprin/fcli fcli version

To that end, I put together a Dockerfile that builds a ~90mb image that provides this functionality; it's included in a PR I just submitted:

#39