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:

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

Creating 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).

Using Private Docker Hub Images

To access private repositories on Docker Hub, <REGISTRY_NAME> should be https://index.docker.io/v1/. The registry domain does not have to be specified in docker run or docker-compose.

Like Docker, Watchtower will use the Docker Hub registry and its credentials when no registry domain is specified. Watchtower will recognize credentials with <REGISTRY_NAME> index.docker.io, but the Docker CLI will not.

Using a Private Registry on a Local Host

To use a private registry hosted locally, make sure to correctly specify the registry host in both config.json and the docker run command or docker-compose file. Valid hosts are localhost[:PORT], HOST:PORT, or any multi-part domain.name or IP-address with or without a port.

Examples:

• localhost -> localhost/myimage
• 127.0.0.1 -> 127.0.0.1/myimage:mytag
• host.domain -> host.domain/myorganization/myimage
• other-lan-host:80 -> other-lan-host:80/imagename:latest

The required auth string can be generated as follows:

echo -n 'username:password' | base64

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 nickfedor/watchtower

Sharing 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 nickfedor/watchtower

When creating the Watchtower container via Docker Compose, use the following lines:

services:
  watchtower:
    image: nickfedor/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.

For example:

services:
  watchtower:
    image: nickfedor/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

Here is an example implementation for use with the amazon-ecr-credential-helper:

Use the Dockerfile below to build the amazon-ecr-credential-helper image in a volume that may be mounted onto your Watchtower container.

1. Create the Dockerfile:

   FROM golang:1.25
   
   ENV GO111MODULE off
   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 <AWS_ACCOUNT_ID> placeholders with your AWS Account ID and <AWS_ECR_REGION> with your AWS ECR Region:

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

   services:
    # Check for new images and restart things if a new image exists
    # for any of our containers.
    watchtower:
      image: nickfedor/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. 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. This may be able to run in 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.

   Then, mount the ~/.aws directory to / in the container.