deploy: uncompress zar when uploading

The middleware can only handle user images in the form of uncompressed
tarballs. Trying to mount a tar.gz file resulted in a silent error.

The error was accidentally introduced in 8c496e19 when I rewrote the
code to use zar.json to produce a tarball.
Merge pull request #38 from mgeisler/uncompress-upload

deploy: uncompress zar when uploading