[WIP] Run rainbow example in Docker

[johnmccabe]

This PR adds a multi-stage Dockerfile which builds the rainbow example on arm32v6/alpine.

To build run:

cd examples/rainbow/
docker build -t johnmccabe/mote-rainbow .

To run the container you either need to pass the path to the Mote devices port.

docker run --device=/dev/ttyACM0 johnmccabe/mote-rainbow

or, run with --privileged.

docker run --privileged johnmccabe/mote-rainbow