link:https://docs.docker.com/registry/spec/auth/[Docker Registry V2 Authentication] is an OIDC-Like protocol used to authenticate users against a Docker registry. {project_name}'s implementation of this protocol allows for a {project_name} authentication server to be used by a Docker client to authenticate against a registry. While this protocol uses fairly standard token and signature mechanisms, it has a few wrinkles that prevent it from being treated as a true OIDC implementation. The largest deviations include a very specific JSON format for requests and responses as well as the ability to understand how to map repository names and permissions to the OAuth scope mechanism.
The link:https://docs.docker.com/registry/spec/auth/token/[Docker API documentation] best describes and illustrates this process, however a brief summary will be given below from the perspective of the {project_name} authentication server.
* The flow begins when the Docker client requests a resource from the Docker registry. If the resource is protected and no auth token is present in the request, the Docker registry server will respond to the client with a 401 + some information on required permissions and where to find the authorization server.
* The Docker client will construct an authentication request based on the 401 response from the Docker registry. The client will then use the locally cached credentials (from a previously run `docker login` command) as part of a link:https://tools.ietf.org/html/rfc2617[HTTP Basic Authentication] request to the {project_name} authentication server.
* The {project_name} authentication server will attempt to authenticate the user and return a JSON body containing an OAuth-style Bearer token.
* The Docker client will get the bearer token from the JSON response and use it in the Authorization header to request the protected resource.
* When the Docker registry receives the new request for the protected resource with the token from the {project_name} server, the registry validates the token and grants access to the requested resource (if appropriate).
