Don't compress the archive made from git's output.

Commit 894d4a23fba made BuildFile use TarSum, which doesn't support
compressed tar archives.
This breaks builds from git url, which compressed it with bzip2.
Instead, just pass it uncompressed.

Docker-DCO-1.0-Signed-off-by: Paul Lietar <paul@lietar.net> (github: plietar)

AUTHORS

@@ -148,6 +148,7 @@ odk- <github@odkurzacz.org>
 Pascal Borreli <pascal@borreli.com>
 Paul Bowsher <pbowsher@globalpersonals.co.uk>
 Paul Hammond <paul@paulhammond.org>
+Paul Liétar <paul@lietar.net>
 Paul Nasrat <pnasrat@gmail.com>
 Phil Spitler <pspitler@gmail.com>
 Piotr Bogdan <ppbogdan@gmail.com>

api.go

@@ -929,7 +929,7 @@ func postBuild(srv *Server, version float64, w http.ResponseWriter, r *http.Requ
 			return fmt.Errorf("Error trying to use git: %s (%s)", err, output)
 		}
 
-		c, err := archive.Tar(root, archive.Bzip2)
+		c, err := archive.Tar(root, archive.Uncompressed)
 		if err != nil {
 			return err
 		}