Make push optional and default to False

Let's optimize the defaults for good commandline usage rather
than for binderhub usage

repo2docker/app.py

@@ -92,11 +92,20 @@ class Repo2Docker(Application):
         """
     )
 
+    push = Bool(
+        False,
+        config=True,
+        help="""
+        If the image should be pushed after it is built.
+        """
+    )
+
     aliases = Dict({
         'repo': 'Repo2Docker.repo',
         'ref': 'Repo2Docker.ref',
         'image': 'Repo2Docker.output_image_spec',
         'clean': 'Repo2Docker.cleanup_checkout',
+        'push': 'Repo2Docker.push',
         'f': 'Repo2Docker.config_file',
     })
 
@@ -174,11 +183,15 @@ class Repo2Docker(Application):
             self.log.error('Could not figure out how to build this repository! Tell us?', extra=dict(phase='failed'))
             sys.exit(1)
 
+        if self.push:
             # Build a progress setup for each layer, and only emit per-layer info every 1.5s
             layers = {}
             last_emit_time = time.time()
             for line in client.push(self.output_image_spec, stream=True):
                 progress = json.loads(line.decode('utf-8'))
+                if 'error' in progress:
+                    self.log.error(progress['error'], extra=dict(phase='failed'))
+                    sys.exit(1)
                 if 'id' not in progress:
                     continue
                 if 'progressDetail' in progress and progress['progressDetail']:
@@ -189,6 +202,7 @@ class Repo2Docker(Application):
                     self.log.info('Pushing image', extra=dict(progress=layers, phase='pushing'))
                     last_emit_time = time.time()
 
+
         if self.cleanup_checkout:
             shutil.rmtree(checkout_path)