Keep tag by regex

Adds `--keep-regex` paramiter which can take multiple python stdlib re
expressions and keep matching tags from entering the deletion set.

Example paramiter:
 --keep-regex '.*:latest' 'ubuntu.*'

This example will ingore all tags starting with "ubuntu" and all images ending in
":latest".

Author: Dylan Grafmyre

dockerrotate/main.py

@@ -3,6 +3,7 @@
 """
 from argparse import ArgumentParser
 from datetime import timedelta
+import re
 
 from dateutil import parser
 from docker import Client
@@ -40,6 +41,13 @@ def parse_args():
         default=3,
         help="Keep this many images of each kind",
     )
+    parser.add_argument(
+        "--keep-regex",
+        type=str,
+        nargs='*',
+        default=list(),
+        help="Python regex of tag names to keep.",
+    )
     parser.add_argument(
         "--only",
         "-o",
@@ -167,6 +175,9 @@ def clean_images(client, args):
             image_name = normalize_tag_name(tag)
             if args.only and args.only != image_name:
                 continue
+            if any([re.match(pattern, tag) for pattern in args.keep_regex]):
+                continue
+
             images_by_name.setdefault(image_name, set()).add(image["Id"])
 
     for image_name, image_ids in images_by_name.items():