Pulling cached Docker Hub images

Container Registry caches frequently-accessed public Docker Hub images on mirror.gcr.io. You can configure the Docker daemon to use a cached public image if one is available, or pull the image from Docker Hub if a cached copy is unavailable.

Google Cloud services such as Cloud Build and Google Kubernetes Engine automatically check for cached images before attempting to pull an image from Docker Hub.

Cached images at mirror.gcr.io are:

  • Stored in a repository that Google manages.
  • More insulated from Docker Hub outages.
  • Easily integrated with the Google Cloud ecosystem.
  • Kept in sync with Docker Hub.

Configuring the Docker daemon

To configure your Docker daemon to pull images from the Container Registry cache:

CLI

  1. Configure the daemon in one of the following ways:

    • To configure the Docker daemon automatically on startup, set the following value in /etc/docker/daemon.json

      {
  "registry-mirrors": ["https://mirror.gcr.io"]
}

    • When you start the daemon, pass in the Container Registry hostname:

      dockerd --registry-mirror=https://mirror.gcr.io

    • Add the following line to your /etc/default/docker file:

      DOCKER_OPTS="${DOCKER_OPTS} --registry-mirror=https://mirror.gcr.io"

  2. Restart the Docker daemon.

    • On Linux, run one of the following commands:

      sudo service docker restart

      or

      sudo service docker stop && sudo service docker start

    • On macOS or Windows, run the following command:

      docker-machine restart

Docker UI

  1. Open Docker's Preferences menu.
  2. Click Daemon.

  3. Click Advanced. In the JSON field, add a registry-mirrors key with https://mirror.gcr.io as a value:

    {
  "registry-mirrors" : [
    "https://mirror.gcr.io"
  ]
}

  4. Click Apply & Restart.

To verify that the cache is correctly configured, run:

docker system info

The output should include Registry Mirrors, and should look similar to the following:

Containers: 2
 Running: 0
 Paused: 0
 Stopped: 2
Images: 2
Server Version: 17.03.1-ce
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
...
Registry Mirrors:
 https://mirror.gcr.io

Pulling cached images

Container Registry adds frequently requested images to the cache so they are available for future requests. It also periodically removes images that are no longer requested.

After you configure the Docker daemon to use the Container Registry cache, Docker performs the following steps when you pull a public Docker Hub image with a docker pull command:

  1. The Docker daemon checks the Container Registry cache and fetches the images if it exists. If your daemon configuration includes other Docker mirrors, the daemon checks each one in order for a cached copy of the image.
  2. If the image still isn't found, the Docker daemon fetches the image from the canonical repository on Docker Hub.

Pulling cached images does not count against Docker Hub rate limits. However, there is no guarantee that a particular image will remain cached for an extended period of time. Only obtain cached images on mirror.gcr.io by configuring the Docker daemon. A request to pull directly from mirror.gcr.io will fail if a cached copy of the image does not exist.

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