btrfs-progs: ci: add dockerfile for a musl build test

btrfs-progs: ci: add dockerfile for a musl build test

Build environment with musl as the libc, based on Alpine Linux.

To build the docker image:
- cd travis/images
- cp test-build $dir
- cd $dir
- docker build -t $tag .
- docker push $tag

We're going to have more images that will share the prep script so it
has to be copied to the directory before making the image. No other way
around that.

Signed-off-by: David Sterba <dsterba@suse.com>