Dockerfiles to work with mbed-cli and yotta.

One for the latest boards, supported by Mbed OS 5/6, and one for dinosaurs, running with Mbed OS 2. Some SDKs for the latter are requiring Python 2. That's why we are having another Dockerfile.

# Build the Docker containers for current boards and dinosaurs
docker build -t arm-build-env .
docker build -t arm-build-legacy-env -f Dockerfile.legacy .

# Switch to your project and run one of those
docker run --rm -it -v `pwd`:/app arm-build-env bash
docker run --rm -it -v `pwd`:/app arm-build-legacy-env bash