Allow listing of ACI rootfs by non-root

Also the image ID folder for docker squash.

This method is only called in 2 places: once for the ACI rootfs, and once
for squashing docker images. The previous (0644) permissions could
theoretically have been depended on by someone, but it is doubtful.

fixes #802

formats/tarimageformatter.py

@@ -43,4 +43,6 @@ class TarImageFormatter(object):
     """ Returns TAR file header data for a folder with the given name. """
     info = tarfile.TarInfo(name=name)
     info.type = tarfile.DIRTYPE
+    # allow the directory to be readable by non-root users
+    info.mode = 0755
     return info.tobuf()