Add keep_by_hours option

To be more adaptive to user needs, keep_by_hours option works as the
inverse of the "delete_by_hours" option. "keep_by_hours" will keep
any tag newer than the specified number of hours.

registry.py

@@ -501,7 +501,14 @@ for more detail on garbage collection read here:
 
     parser.add_argument(
         '--delete-by-hours',
-        help=('Will delete all tags that older than specified hours. Be careful!'),
+        help=('Will delete all tags that are older than specified hours. Be careful!'),
+        default=False,
+        nargs='?',
+        metavar='Hours')
+
+    parser.add_argument(
+        '--keep-by-hours',
+        help=('Will keep all tags that are newer than specified hours.'),
         default=False,
         nargs='?',
         metavar='Hours')
@@ -605,6 +612,30 @@ def delete_tags_by_age(registry, image_name, dry_run, hours, tags_to_keep):
     delete_tags(registry, image_name, dry_run, tags_to_delete, tags_to_keep)
 
 
+def get_newer_tags(registry, image_name, hours, tags_list):
+    newer_tags = []
+    print('---------------------------------')
+    for tag in tags_list:
+        image_config = registry.get_tag_config(image_name, tag)
+
+        if image_config == []:
+            print("tag not found")
+            continue
+
+        image_age = registry.get_image_age(image_name, image_config)
+
+        if image_age == []:
+            print("timestamp not found")
+            continue
+
+        if dt.strptime(image_age[:-4], "%Y-%m-%dT%H:%M:%S.%f") >= dt.now() - timedelta(hours=int(hours)):
+            print("Keeping tag: {0} timestamp: {1}".format(
+                tag, image_age))
+            newer_tags.append(tag)
+
+    return newer_tags
+
+
 def main_loop(args):
     global DEBUG
 
@@ -682,10 +713,14 @@ def main_loop(args):
                             layer['blobSum']))
 
         # add tags to "tags_to_keep" list, if we have regexp "tags_to_keep"
-        # entries:
+        # entries or a number of hours for "keep_by_hours":
         keep_tags = []
         if args.keep_tags_like:
             keep_tags.extend(get_tags_like(args.keep_tags_like, tags_list))
+        if args.keep_by_hours:
+            keep_tags.extend(get_newer_tags(registry, image_name,
+                                            args.keep_by_hours, tags_list))
+        keep_tags = list(set(keep_tags))  # Eliminate duplicates
 
         # delete tags if told so
         if args.delete or args.delete_all: