Add --pull argument to pull an updated image

README.md

 If a container uses an argument that's not supported yet, it will be silently
 dropped when rerunning.
 
-
-The following arguments can be used when executing `docker-rerun` to modify
-the resulting container:
-
- * `--image <image>` - changes the image that will be used. You can specify
-   tags (`name:tag`) or digests (`name@digest`) as with `docker run`.
- * `-p <port>` or `--port <port>` - expose additional ports. Same format
-   as `docker run`'s `-p` option.
+## Arguments
+
+The built-in help shows all available arguments. `--dry-run` and `--pull`
+affect the behaviour of `docker-rerun`; other options allow modification
+of the container's parameters.
+
+    usage: docker-rerun [-h] [-d] [--pull] [--image IMAGE] [--port PORT] container
+    
+    Reruns docker containers with different parameters.
+    
+    positional arguments:
+      container             The container to rerun
+    
+    optional arguments:
+      -h, --help            show this help message and exit
+      -d, --dry-run         Don't actually re-run the container, just print what
+                            would happen.
+      --pull                Docker pull the image before re-running the container
+      --image IMAGE         Image to use in place of the original
+      --port PORT, -p PORT  Additional port to expose
 
 ## What's not done yet
 

docker-rerun

     parser.add_argument('-d', '--dry-run', action='store_true',
                         help='Don\'t actually re-run the container, just ' \
                              'print what would happen.')
+    parser.add_argument('--pull', action='store_true',
+                        help='Docker pull the image before re-running the ' \
+                             'container')
 
     mods = modifiers()
     for mod in mods:
         container.command_line(),
     ]
 
+    if args.pull:
+        commands = [['docker', 'pull', container.image]] + commands
+
     if args.dry_run:
         print('Performing dry run for container %s. The following would be ' \
               'executed:' % args.container)