floss: Add docker image for build

Add instructions on how to build with docker and include the dockerfile
used to generate the image used. Also include a python script that can
be run to generate the docker image and build everything.

* `sudo ./docker-build-image.py --tag floss:latest` will build the
  docker image from scratch

* `sudo ./build-in-docker.py` will build Floss in the current
  repository.

Tag: #floss
Bug: 220203992
Test: ./floss/build/docker-build.py and ./build.py inside container
Change-Id: I0888a4e00de2d1f561874d6a555fc847538a131c