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Private registries

Watchtower supports private Docker image registries. In many cases, accessing a private registry requires a valid username and password (i.e., credentials). In order to operate in such an environment, watchtower needs to know the credentials to access the registry.

The credentials can be provided to watchtower in a configuration file called config.json. There are two ways to generate this configuration file:

  • The configuration file can be created manually.
  • Call docker login <REGISTRY_NAME> and share the resulting configuration file.

Create the configuration file manually

Create a new configuration file with the following syntax and a base64 encoded username and password auth string:

{
    "auths": {
        "<REGISTRY_NAME>": {
            "auth": "XXXXXXX"
        }
    }
}

<REGISTRY_NAME> needs to be replaced by the name of your private registry (e.g., my-private-registry.example.org)

The required auth string can be generated as follows:

echo -n 'username:password' | base64

Username and Password for GCloud

For gcloud, we'll use _json_key as our username and the content of gcloudauth.json as the password.

bash echo -n "_json_key:$(cat gcloudauth.json)" | base64 -w0

When the watchtower Docker container is started, the created configuration file (<PATH>/config.json in this example) needs to be passed to the container:

docker run [...] -v <PATH>/config.json:/config.json containrrr/watchtower

Share the Docker configuration file

To pull an image from a private registry, docker login needs to be called first, to get access to the registry. The provided credentials are stored in a configuration file called <PATH_TO_HOME_DIR>/.docker/config.json. This configuration file can be directly used by watchtower. In this case, the creation of an additional configuration file is not necessary.

When the Docker container is started, pass the configuration file to watchtower:

docker run [...] -v <PATH_TO_HOME_DIR>/.docker/config.json:/config.json containrrr/watchtower

When creating the watchtower container via docker-compose, use the following lines:

version: "3.4"
services:
  watchtower:
    image: index.docker.io/containrrr/watchtower:latest
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - <PATH_TO_HOME_DIR>/.docker/config.json:/config.json
  ...

Docker Config path

By default, watchtower will look for the config.json file in /, but this can be changed by setting the DOCKER_CONFIG environment variable to the directory path where your config is located. This is useful for setups where the config.json file is changed while the watchtower instance is running, as the changes will not be picked up for a mounted file if the inode changes. Example usage:

version: "3.4"

services: 
  watchtower:
    image: containrrr/watchtower
    environment:
        DOCKER_CONFIG: /config
    volumes:
      - /etc/watchtower/config/:/config/
      - /var/run/docker.sock:/var/run/docker.sock

Credential helpers

Some private Docker registries (the most prominent probably being AWS ECR) use non-standard ways of authentication. To be able to use this together with watchtower, we need to use a credential helper.

To keep the image size small we've decided to not include any helpers in the watchtower image, instead we'll put the helper in a separate container and mount it using volumes.

Example

Example implementation for use with amazon-ecr-credential-helper:

Use the dockerfile below to build the amazon-ecr-credential-helper, in a volume that may be mounted onto your watchtower container.

  1. Create the Dockerfile (contents below):

    FROM golang:latest
    
    ENV CGO_ENABLED 0
    ENV REPO github.com/awslabs/amazon-ecr-credential-helper/ecr-login/cli/docker-credential-ecr-login
    
    RUN go get -u $REPO
    
    RUN rm /go/bin/docker-credential-ecr-login
    
    RUN go build \
     -o /go/bin/docker-credential-ecr-login \
     /go/src/$REPO
    
    WORKDIR /go/bin/
    

  2. Use the following commands to build the aws-ecr-dock-cred-helper and store it's output in a volume:

    # Create a volume to store the command (once built)
    docker volume create helper 
    
    # Build the container
    docker build -t aws-ecr-dock-cred-helper .
    
    # Build the command and store it in the new volume in the /go/bin directory.
    docker run  -d --rm --name aws-cred-helper \
      --volume helper:/go/bin aws-ecr-dock-cred-helper
    

  3. Create a configuration file for docker, and store it in $HOME/.docker/config.json (replace the placeholders with your AWS Account ID):

    {
       "credsStore" : "ecr-login",
       "HttpHeaders" : {
         "User-Agent" : "Docker-Client/19.03.1 (XXXXXX)"
       },
       "auths" : {
         "<AWS_ACCOUNT_ID>.dkr.ecr.us-west-1.amazonaws.com" : {}
       },
       "credHelpers": {
         "<AWS_ACCOUNT_ID>.dkr.ecr.us-west-1.amazonaws.com" : "ecr-login"
       }
    }
    

  4. Create a docker-compose file (as an example) to help launch the container:

    version: "3.4"
    services:
     # Check for new images and restart things if a new image exists
     # for any of our containers.
     watchtower:
       image: containrrr/watchtower:latest
       volumes:
         - /var/run/docker.sock:/var/run/docker.sock
         - .docker/config.json:/config.json
         - helper:/go/bin
       environment:
         - HOME=/
         - PATH=$PATH:/go/bin
         - AWS_REGION=us-west-1
    volumes:
     helper: 
       external: true
    

A few additional notes:

  1. With docker-compose the volume (helper, in this case) MUST be set to external: true, otherwise docker-compose will preface it with the directory name.
  2. Note that "credsStore" : "ecr-login" is needed - and in theory if you have that you can remove the credHelpers section
  3. I have this running on an EC2 instance that has credentials assigned to it - so no keys are needed; however, you may need to include the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables as well.
  4. An alternative to adding the various variables is to create a ~/.aws/config and ~/.aws/credentials files and place the settings there, then mount the ~/.aws directory to / in the container.