Use str.partition() for --label argument parsing

repo2docker/__main__.py

@@ -263,11 +263,8 @@ def make_r2d(argv=None):
         r2d.appendix = args.appendix
 
     for l in args.labels:
-        if "=" in l:
+        key, _, val = l.partition("=")
-            key, val = l.split("=", 1)
+        r2d.labels[key] = val
-            r2d.labels[key] = val
-        else:
-            r2d.labels[l] = ""
 
     for a in args.build_args:
         key, _, val = a.partition("=")