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docker-credential-env plugin

A Docker credentials plugin that sources Docker credentials from environment variables. This is an alternative to using docker login directly.

Some CI providers use environment variables to communicate the configuration of Docker Hub. In these cases, instead of performing a docker login when bootstrapping the agent for use, one can instead build this helper into the agent image.

This has some benefits:

  • Reliability: the login will fail only when running an action on an image (pull, push etc). If the step has no docker actions, it will not be affected. Many CI systems now allow agents to login to Docker Hub all the time, regardless of the actions that will be performed. Using this plugin means that only actions that attempt to use the failing credentials will cause a failure.
  • Performance: since login is only attempted when the credentials are required, agents avoid an unnecessary setup step. This may seem insignificant, but it can add up in aggregate.

When environment variables are set appropriately, docker will use the credentials as needed.

Configuration

Note

The Docker CLI (docker) is the process that calls credential helpers, not the daemon. All environment variables (PATH and others) need to be set for the process calling Docker, and the executing user needs to be able to execute the helper binary.

Installation

The binary needs to be added to the local PATH in order to be accessible to Docker for use. The Docker CLI calls the helper (not the daemon), so the executing user's PATH is used.

Registry configuration

The plugin uses environment variables in a particular format to supply credentials to the Docker process. These environment variables need to be present in the process executing the docker CLI command.

All environment variables have the form: DOCKER_CREDENTIALS_ENV_<REGISTRYURL>_<USER|PASSWORD>, where:

  • REGISTRY_URL identifies the URL that the credentials are for. This is the registry URL (really just the host) as configured in config.json, with no leading scheme and no trailing slash. For the ECR public registry, the registry host is public.ecr.aws, so the environment variable pair will include PUBLIC_ECR_AWS in the name. The special value DEFAULT can be used for the Docker Hub registry.
  • <USER|PASSWORD>: credentials are supplied as a pair of variables: the USER and the PASSWORD.

Fail silently

If your environment should fail quietly if the authentication variables are not present, set the DOCKER_CREDENTIALS_ENV_OPTIONAL variable to true. When this variable is set, the plugin will write an error indicating that the credentials weren't found, but will not return an error to Docker.

Examples

Docker Hub

Environment:

export DOCKER_CREDENTIALS_ENV_INDEX_DOCKER_IO_USER=dockerhubusername
export DOCKER_CREDENTIALS_ENV_INDEX_DOCKER_IO_PASSWORD=userapikey

~/.docker/config.json fragment:

{
 "credHelpers": {
    "https://index.docker.io/v1/": "env"
  },
}

Important

Unlike other registries, the default registry must be specified in config.json as a full URL. This quirk is only relevant to the Docker Hub registry.

ECR public registry

Environment:

export DOCKER_CREDENTIALS_ENV_PUBLIC_ECR_AWS_USER=AWS
export DOCKER_CREDENTIALS_ENV_PUBLIC_ECR_AWS_PASSWORD=password-from-aws-cli

~/.docker/config.json fragment:

{
 "credHelpers": {
    "public.ecr.aws": "env"
  },
}