Make ACI generation consistent across calls

This will ensure that no matter which signature we write for the generated ACI, it is correct for that image.

formats/aci.py

@@ -1,5 +1,6 @@
 import json
 import re
+import calendar
 
 from uuid import uuid4
 
@@ -17,16 +18,21 @@ class ACIImage(TarImageFormatter):
 
   def stream_generator(self, namespace, repository, tag, synthetic_image_id,
                        layer_json, get_image_iterator, get_layer_iterator, get_image_json):
+    image_mtime = 0
+    created = next(get_image_iterator()).created
+    if created is not None:
+      image_mtime = calendar.timegm(created.utctimetuple())
+
     # ACI Format (.tar):
     #   manifest - The JSON manifest
     #   rootfs - The root file system
 
     # Yield the manifest.
-    yield self.tar_file('manifest', self._build_manifest(namespace, repository, tag, layer_json,
-                                                         synthetic_image_id))
+    manifest = self._build_manifest(namespace, repository, tag, layer_json, synthetic_image_id)
+    yield self.tar_file('manifest', manifest, mtime=image_mtime)
 
     # Yield the merged layer dtaa.
-    yield self.tar_folder('rootfs')
+    yield self.tar_folder('rootfs', mtime=image_mtime)
 
     layer_merger = StreamLayerMerger(get_layer_iterator, path_prefix='rootfs/')
     for entry in layer_merger.get_generator():
@@ -187,7 +193,6 @@ class ACIImage(TarImageFormatter):
 
       env_vars.append(pieces)
 
-
     manifest = {
       "acKind": "ImageManifest",
       "acVersion": "0.6.1",