dockerTools: private registry support

* authorization token is optional
* registry url is taken from X-Docker-Endpoints header
* pull.sh correctly resumes partial layer downloads
* detjson.py does not fail on missing keys

pkgs/build-support/docker/detjson.py

@@ -24,9 +24,11 @@ SAFEDELS["container_config"] = SAFEDELS["config"]
 
 def makedet(j, safedels):
     for k,v in safedels.items():
+        if k not in j:
+            continue
         if type(v) == dict:
             makedet(j[k], v)
-        elif k in j and j[k] == v:
+        elif j[k] == v:
             del j[k]
 
 def main():