docker_build.py: byte-to-string conv for Python 3

docker_build.py

@@ -160,7 +160,7 @@
     buildargs = dict()
     if os.path.exists('.hg'):
         buildargs['REVISION'] = check_output(
-            'hg identify -i'.split()).split()[0]
+            'hg identify -i'.split()).split()[0].decode('utf-8')
     buildargs['CREATED'] = datetime.datetime.now().isoformat()
     if nmspc.build_arg:
         for arg in nmspc.build_arg:
@@ -202,7 +202,7 @@
     # this is for python docker 1.8-1.9
     # TODO add compatibility with newer python docker
     for line in builder:
-        d = json.loads(line)
+        d = json.loads(line.decode('utf-8'))
         if 'stream' in d:
             logging.info(d['stream'])
         if 'errorDetail' in d: