Enable Docker builds on Windows

#391 added the ability to use Docker to control Linux AppImage builds.

In theory, this should allow a Linux build to be compiled on any computer that supports Docker. macOS already supports this; however, there is a problem with Windows support.

Some of the arguments passed to the Docker context are paths or partial paths; these need to be converted from Windows separators to Unix separators when they're passed as arguments to Docker.

As a result of these problems, Docker support on Windows [has been disabled](https://github.com/beeware/briefcase/pull/391/commits/b5e719023dd36fc6a1a1ef670131df833d737db1). However, it would be desirable to re-enable this support.