Add support for modifiers.

Initially just support --image to change the image.

README.md

 Want to update to a newer image, or add a missing port publication?
 docker-rerun's got you covered.
 
-## How to use it
+## Usage
 
 In the most basic usage, you pass in a container name and it will be
 stopped, deleted and recreated:
 
     $ ./docker-rerun apache
 
+You can also pass additional arguments to modify aspects of the container
+when it's rerun. For example, to change the image:
+
+    $ ./docker-rerun --image nginx:latest webserver
+
 To check what exactly is going to be performed beforehand, use the --dry-run
 option:
 
 
 ## What's supported
 
-At present docker-rerun supports a small number of commonly used arguments:
+At present docker-rerun supports copying a number of commonly used arguments:
 
     * Commands (trailing arguments)
     * Environment variables (-e/--env)
     * User switching (-u/--user)
     * Volumes (-v/--volume, and --volumes-from)
 
+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`.
+
 ## What's not done yet
 
 Many other command line arguments:
     * Permissions and policies
     * Advanced networking options
 
-Additional options to allow mutating the container config when rerunning.
-For example:
-
-    $ ./docker-rerun --image nginx:1.11.1 nginx
-
-Should replace the previously used image with the one specified.
+More options to allow mutating the container config when rerunning.
 

docker-rerun

 Each function named `handle_*` handles one configuration option,
 reading the relevant information from the inspect output and adding
 the relevant command line flags to the config.
+
+Each function named `modify_*` allows the user to modify the
+configuration in some manner. Modify functions are called twice:
+once with an ArgumentParser, and subsequently with the parsed
+arguments and the container object.
 """
 
 import argparse
                                  lambda c: c['HostConfig']['VolumesFrom'])
 
 
+def modify_image(parser=None, args=None, container=None):
+    """Allows the image (name, version, etc) to be modified in one go."""
+    if parser:
+        parser.add_argument('--image',
+                            help='Image to use in place of the original')
+    elif args.image:
+        container.image = args.image
+
+
 def functions():
     """Lists all functions defined in this module.
 
     return [func for (name, func) in functions() if name.startswith('handle_')]
 
 
+def modifiers():
+    """Lists all modifiers defined in this module.
+
+    Returns:
+        list of function: All modifiers (modify_* funcs) in this module.
+    """
+    return [func for (name, func) in functions() if name.startswith('modify_')]
+
+
 def main():
     """Script entry point."""
     parser = argparse.ArgumentParser(description='Reruns docker containers ' \
     parser.add_argument('-d', '--dry-run', action='store_true',
                         help='Don\'t actually re-run the container, just ' \
                              'print what would happen.')
+
+    mods = modifiers()
+    for mod in mods:
+        mod(parser=parser)
+
     args = parser.parse_args()
     container_info = docker_inspect(args.container, 'container')
     image_info = docker_inspect(container_info['Config']['Image'], 'image')
     for handler in handlers():
         handler(container)
 
+    for mod in mods:
+        mod(args=args, container=container)
+
     commands = [
         ['docker', 'stop', args.container],
         ['docker', 'rm', args.container],

test.py

     yield check, ['docker', 'run', '--link=testA:bar', '--link=testB', '--name=test123', '-d', 'hello-world']
 
 
+@with_setup(setup, teardown)
+def test_modifiers():
+    yield (check,
+           ['docker', 'run', '--name=test123', '-d', 'hello-world'],
+           ['--image', 'tutum/hello-world'],
+           ['docker', 'run', '--name=test123', '-d', 'tutum/hello-world'])
+    yield (check,
+           ['docker', 'run', '--name=test123', '-d', 'hello-world'],
+           ['--image', 'hello-world:latest'],
+           ['docker', 'run', '--name=test123', '-d', 'hello-world:latest'])
+
+
 @with_setup(setup_each, teardown_each)
-def check(command):
+def check(command, args=[], expected=None):
     _run(command)
-    output = subprocess.check_output(['./docker-rerun', '--dry-run', 'test123'])
+    output = subprocess.check_output(['./docker-rerun', '--dry-run', 'test123'] + args)
     output = output.decode('utf-8').strip().splitlines()
-    assert output[3] == ' '.join(command), 'Expected "%s" but got "%s"' % (' '.join(command), output[3])
+    expected = ' '.join(expected or command)
+    assert output[3] == expected, 'Expected "%s" but got "%s"' % (expected, output[3])