[RFD] add configuration option to add `host.docker.internal` by default

Pull request https://github.com/moby/moby/pull/40007 implemented a "magic" host-gateway value to allow users to add host.docker.internal to /etc/hosts in containers, mapping it to the gateway IP address (the IP address itself can be configured in the daemon configuration).

With this feature, users running dockerd on Linux can get parity with Docker for Mac / Docker for Windows to have host.docker.internal resolve inside containers.

Currently, this option is "opt-in" on Linux; containers will not be able to resolve host.docker.internal unless they are created with --add-host host.docker.internal:host-gateway

Proposal

Users may want to have the host.docker.internal host added automatically for every container created (similar to how it's currently added automatically on Docker Desktop (Docker for Mac / Docker for Windows).

I suggest to:

• add an option to the CLI configuration file (~/.docker/config.json) that allows configuring a default (set of) custom hosts for new containers
• possibly (t.b.d.) an environment variable to configure this

The configuration option could potentially be used by the Docker Desktop team, as a replacement for the current (proprietary) implementation; doing so would make the implementation on Linux and Docker Desktop identical (which would make it less confusing).

Start by reviewing the Docker CLI configuration at ~/.docker/config.json and tracing how --add-host host.docker.internal:host-gateway is passed when containers are created. Clarify the proposed default custom-host format, its interaction with Docker Desktop, and whether an environment variable is required; done means an agreed design and corresponding implementation scope.