docker: require btrfs-progs >= 3.16.1 for master branch

Current master (future 1.4 release) requires btrfs/version.h which is
only available as-of 3.16.1 and later.
docker: configure driver via environment variable

Docker supports configuring the storage driver via an environment
variable. Using this makes it much easier to alter with a drop-in.
Merge pull request #1000 from marineam/docker

docker: require btrfs-progs >= 3.16.1 for master branch