Configuration for docker/docker-credential-helpers

The configuration of docker/docker-credential-helpers primarily involves defining credential storage mechanisms for Docker. Below is a step-by-step guide for configuring the development environment using various options available in the repository.

Setting Up Configuration

Utilize the Environment Variables:

The credential helper can be configured using environment variables that you can set directly in your shell or in your project’s environment configuration files. For instance, if you wish to define a helper like osxkeychain, you would configure it like so:
```
export DOCKER_CREDENTIAL_HELPER=osxkeychain
```
This variable indicates that Docker should use the macOS Keychain for storing credentials.
Configuration via Docker Config File:

Another way to configure the credential helper is to specify it in the Docker configuration file located at $HOME/.docker/config.json. Below is an example configuration that specifies different credential helpers depending on the repository:
```
{
  "credHelpers": {
    "https://index.docker.io/v1/": "osxkeychain",
    "my-custom-registry.com": "pass"
  }
}
```
This file tells Docker to use the osxkeychain for the default Docker registry and pass for my-custom-registry.com.

Custom Credential Helper Implementation:

If you need to implement a custom credential helper, you can define your struct that implements the necessary functions (Get, Store, Erase) defined by the Docker credential helpers interface. Below is a basic outline of how this might look in Go:

package main

import (
    "github.com/docker/docker-credential-helpers/credentials"
)

type MyCredentialHelper struct{}

func (m *MyCredentialHelper) Get(serverURL string) (string, string, error) {
    // Retrieve and return the username and password
}

func (m *MyCredentialHelper) Store(cred *credentials.Credential) error {
    // Store the credentials
}

func (m *MyCredentialHelper) Erase(serverURL string) error {
    // Erase the credentials stored for the URL
}

func main() {
    // Code to initiate and register the helper with Docker
}

Ensure that the built binary is named according to the default Docker naming conventions (e.g., docker-credential-mycustomhelper) to make it discoverable by Docker.

Using Makefile for Build Configuration:

If you are managing the build using a Makefile, you can define specific rules for your credential helper. Below is a sample Makefile entry you might consider:
```
all: build

build:
    go build -o docker-credential-mycustomhelper github.com/docker/docker-credential-helpers/myhelper

clean:
    rm -f docker-credential-mycustomhelper
```
In this Makefile, the target build compiles the helper from the source code and names the output appropriately.
Dockerfile for Containerized Environment:

In scenarios where the helper is containerized, you can utilize a Dockerfile for managing the environment:
```
FROM golang:1.17

WORKDIR /app

COPY . .

RUN go build -o docker-credential-mycustomhelper github.com/docker/docker-credential-helpers/myhelper

ENTRYPOINT ["/app/docker-credential-mycustomhelper"]
```
This Dockerfile sets up a Go build environment for the helper within a container, outputting the binary for use.
Testing Your Setup:

After setting up the configuration and building your helper, it is essential to validate it with Docker. You can do this as follows:
```
docker login my-custom-registry.com
```
Observing the behavior of the login command will confirm whether your credential helper configurations are correctly utilized.

For further details on the namespace and credential helpers, refer to the docker/docker-credential-helpers GitHub repository here.