Always add parent directories of files to snapshots.

During a snapshot, when a file changed and not its parent directories,
the parent directories weren't added to the layer. This is inconsistent
with Docker's behavior which always add parent directories to the layer.
In some edge-cases, it could lead to problems with docker considering
that parent directories where owned by root in forthcoming layers
although they shouldn't (see #1163).

Also, Docker seems to be POSIX compliant regarding the name of
directories in the archive, which always have a slash appended. This
commit also fixes this.

Fixes #1163

pkg/util/tar_util.go

@@ -84,6 +84,10 @@ func (t *Tar) AddFileToTar(p string) error {
 		hdr.Name = p
 	}
 
+	if hdr.Typeflag == tar.TypeDir && !strings.HasSuffix(hdr.Name, "/") {
+		hdr.Name = hdr.Name + "/"
+	}
+
 	// rootfs may not have been extracted when using cache, preventing uname/gname from resolving
 	// this makes this layer unnecessarily differ from a cached layer which does contain this information
 	hdr.Uname = ""