Add a better Docker build script + update Dockerfile (#4222)

Add a better Docker build script + update Dockerfile (#4222)

* Add a Docker build script

* Add usage and error messages

* Add -r to reads

Thanks mechmerlin

* Add keyboard:keymap form, improve script

* Add target argument, change usage forms in script

* Add check for more than 3 args in keyboard:keymap:target form

* Change Docker base image to debian, use community repo

This matches what qmk_compiler uses (https://github.com/qmk/qmk_compiler/blob/master/Dockerfile#L1).

I've removed the maintainer as we now have a community build on Docker Hub (https://hub.docker.com/r/qmkfm/qmk_firmware). This Dockerfile will also be maintained by the community.

* Change build command format to keyboard:keymap

* Call make directly in container run command

* Simplify script, remove 3-arg form

* Add COPY to Dockerfile so images are usable in and of themselves

Also change WORKDIR from /qmk to /qmk_firmware

* Add USB pass-through for Linux and docker-machine hosts

* Read directly into variables instead of array

* Alphabetically sort dependencies in Dockerfile

* Set executable bit on util/docker_build.sh

* Update Docker docs

* Add warning about Docker on Windows

* Expand comment in docs

* Check docker-machine exit code instead of string

* Only match --help with whole arguments

* Make script POSIX-compliant

* Convert script indentation to tabs