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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

ℹ️ Username and Password for GCloud

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

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"
[...]
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
[...]

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:

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/

and the docker-compose definition:

version: "3"

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
      - helper:/go/bin
    environment:
      - HOME=/
      - PATH=$PATH:/go/bin
      - AWS_REGION=<AWS_REGION>
      - AWS_ACCESS_KEY_ID=<AWS_ACCESS_KEY>
      - AWS_SECRET_ACCESS_KEY=<AWS_SECRET_ACCESS_KEY>
volumes:
  helper: {}

and for <PATH_TO_HOME_DIR>/.docker/config.json:

  {
    "HttpHeaders" : {
      "User-Agent" : "Docker-Client/19.03.1 (XXXXXX)"
    },
    "credsStore" : "osxkeychain",
    "auths" : {
      "xyzxyzxyz.dkr.ecr.eu-north-1.amazonaws.com" : {},
      "https://index.docker.io/v1/": {}
    },
    "credHelpers": {
      "xyzxyzxyz.dkr.ecr.eu-north-1.amazonaws.com" : "ecr-login",
      "index.docker.io": "osxkeychain"
    }
  }

Note: osxkeychain can be changed to your preferred credentials helper.