Walk file tree in order for deterministic builds

Python's `os.walk` does not order the lists of files and directories it returns, so in order to produce a consistent build context tarfile, they should be ordered.

docker/utils/utils.py

@@ -72,7 +72,8 @@ def tar(path, exclude=None):
             fnames = [name for name in filenames
                       if not fnmatch_any(os.path.join(relpath, name),
                                          exclude)]
-        for name in fnames:
+        dirnames.sort()
+        for name in sorted(fnames):
             arcname = os.path.join(relpath, name)
             t.add(os.path.join(path, arcname), arcname=arcname)
         for name in dirnames: