First of all, just wanted to say thanks for putting this library together. I've been using it for a few weeks and it works great. I'm primarily using it to programmatically build, tag and push docker images on an Amazon EC2 instance to a private ECR registry.

I've added a few pieces of functionality in my fork that I'd be happy to merge back in but I wanted to run the implementations by you first.

Easy stuff:

1. Tagging images.

tagImage :: forall m. MonadUnliftIO m => T.Text -> T.Text -> Maybe Tag -> DockerT m (Either DockerError ())
tagImage name repo maybeTag = requestUnit POST (TagImageEndpoint name repo maybeTag)

2. Pushing images.

pushImage :: forall m. MonadUnliftIO m => T.Text ->  Maybe Tag -> DockerT m (Either DockerError ())
pushImage name maybeTag = requestUnit POST (PushImageEndpoint name maybeTag)

Medium Stuff

1. MonadUnliftIO. How would you feel about switching the constraints on the Api.hs functions from (MonadIO m, MonadMask m) to just MonadUnliftIO m? I noticed you have a #if MIN_VERSION_http_conduit(2,3,0) check in Http.hs for backwards compatibility with older snapshots.

Bigger Stuff

1. Authentication. I added X-REGISTRY-CONFIG for building images and X-REGISTRY-AUTH for tagging and pushing images.
2. Credentials helpers. AWS and other platforms have different methods for doing docker authentication that are a little more complex than just the docker username and password. It seems like the preferred method for accomplishing this is described here. The gist of it is that you have a section in your ~/.docker/config.json which specifies a binary to run to get registry creds. The API that I'm thinking of for accessing these would let the user specify how to do auth in DockerClientOpts:

data DockerAuthStrategy = NoAuthStrategy
                        | ExplicitStrategy RegistryConfig
                        | DiscoverStrategy

NoAuthStrategy obviously uses no authentication. ExplicitStrategy let's you explicitly pass in a RegistryConfig mapping. DiscoverStrategy will look for a credsStore field in the ~/.docker/config.json file and use the binary it points to to automatically retrieve the auth parameters for each request. The DiscoverStrategy is inspired by amazonka

Let me know what you think!