Add daemon flag to specify public registry mirrors

Adds support for a --registry-mirror=scheme://<host>[:port]
daemon flag. The flag may be present multiple times. If
provided, mirrors are prepended to the list of endpoints used
for image pull. Note that only mirrors of the public
index.docker.io registry are supported, and image/tag resolution
is still performed via the official index.

Docker-DCO-1.1-Signed-off-by: Tim Smith <timbot@google.com> (github: timbot)

docs/man/docker.1.md

@@ -64,6 +64,9 @@ unix://[/path/to/socket] to use.
 **-p**=""
   Path to use for daemon PID file. Default is `/var/run/docker.pid`
 
+**--registry-mirror=<scheme>://<host>
+  Prepend a registry mirror to be used for image pulls. May be specified multiple times.
+
 **-s**=""
   Force the Docker runtime to use a specific storage driver.
 

docs/sources/articles.md

@@ -12,3 +12,4 @@
  - [Automatically Start Containers](host_integration/)
  - [Link via an Ambassador Container](ambassador_pattern_linking/)
  - [Increase a Boot2Docker Volume](b2d_volume_resize/)
+ - [Run a Local Registry Mirror](registry_mirror/)

docs/sources/articles/registry_mirror.md

@@ -0,0 +1,83 @@
+page_title: Run a local registry mirror
+page_description: How to set up and run a local registry mirror
+page_keywords: docker, registry, mirror, examples
+
+# Run a local registry mirror
+
+## Why?
+
+If you have multiple instances of Docker running in your environment
+(e.g., multiple physical or virtual machines, all running the Docker
+daemon), each time one of them requires an image that it doesn't have
+it will go out to the internet and fetch it from the public Docker
+registry. By running a local registry mirror, you can keep most of the
+image fetch traffic on your local network.
+
+## How does it work?
+
+The first time you request an image from your local registry mirror,
+it pulls the image from the public Docker registry and stores it locally
+before handing it back to you. On subsequent requests, the local registry
+mirror is able to serve the image from its own storage.
+
+## How do I set up a local registry mirror?
+
+There are two steps to set up and use a local registry mirror.
+
+### Step 1: Configure your Docker daemons to use the local registry mirror
+
+You will need to pass the `--registry-mirror` option to your Docker daemon on
+startup:
+
+    docker --registry-mirror=http://<my-docker-mirror-host> -d
+
+For example, if your mirror is serving on `http://10.0.0.2:5000`, you would run:
+
+    docker --registry-mirror=http://10.0.0.2:5000 -d
+
+**NOTE:**
+Depending on your local host setup, you may be able to add the
+`--registry-mirror` options to the `DOCKER_OPTS` variable in
+`/etc/defaults/docker`.
+
+### Step 2: Run the local registry mirror
+
+You will need to start a local registry mirror service. The
+[`registry` image](https://registry.hub.docker.com/_/registry/) provides this
+functionality. For example, to run a local registry mirror that serves on
+port `5000` and mirrors the content at `registry-1.docker.io`:
+
+    docker run -p 5000:5000 \
+        -e STANDALONE=false \
+        -e MIRROR_SOURCE=https://registry-1.docker.io \
+        -e MIRROR_SOURCE_INDEX=https://index.docker.io registry
+
+## Test it out
+
+With your mirror running, pull an image that you haven't pulled before (using
+`time` to time it):
+
+    $ time docker pull node:latest
+    Pulling repository node
+    [...]
+    
+    real   1m14.078s
+    user   0m0.176s
+    sys    0m0.120s
+
+Now, remove the image from your local machine:
+
+    $ docker rmi node:latest
+
+Finally, re-pull the image:
+
+    $ time docker pull node:latest
+    Pulling repository node
+    [...]
+    
+    real   0m51.376s
+    user   0m0.120s
+    sys    0m0.116s
+
+The second time around, the local registry mirror served the image from storage,
+avoiding a trip out to the internet to refetch it.

docs/sources/reference/commandline/cli.md

@@ -71,6 +71,7 @@ expect an integer, and they can only be specified once.
       --mtu=0                                    Set the containers network MTU
                                                    if no value is provided: default to the default route MTU or 1500 if no default route is available
       -p, --pidfile="/var/run/docker.pid"        Path to use for daemon PID file
+      --registry-mirror=[]                       Specify a preferred Docker registry mirror
       -s, --storage-driver=""                    Force the Docker runtime to use a specific storage driver
       --selinux-enabled=false                    Enable selinux support. SELinux does not presently support the BTRFS storage driver
       --storage-opt=[]                           Set storage driver options