Expose a handler and endpoint in the Agent API to list all running devcontainers.

Running devcontainers will be identified using a combination of docker ps -q and docker inspect commands.

If the docker binary is not present in the agent's $PATH or the agent lacks required permissions, log the error and return the error on subsequent requests to the endpoint.

Also expose an endpoint in the Coder API to allow directly querying this agent API endpoint, similar to the "listening ports" endpoint.

For now, only support Docker via docker CLI.