Add support for tcp:// DOCKER_HOST

The Docker daemon that I am using is running remotely, and I connect to
it via Tailscale tunnel, which means that my DOCKER_HOST is set to
tcp://100.113.182.91:2375

This change makes Dagger work with my setup. It's been running well for
me for a few weeks now, this started as https://github.com/thechangelog/changelog.com/pull/395

I think there is an opportunity to add support for ssh:// & file://,
but I am keeping this first addition small on purpose.

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>
Signed-off-by: Sam Alba <samalba@users.noreply.github.com>

stdlib/docker/command.cue

@@ -30,6 +30,10 @@ import (
 		keyPassphrase?: dagger.#Secret @dagger(input)
 	}
 
+	// Connect via DOCKER_HOST, supports tcp://
+	// TODO: Consider refactoring to support ssh:// & even file://
+	host?: string @dagger(input)
+
 	// Command to execute
 	command: string
 
@@ -203,6 +207,9 @@ import (
 						FINGERPRINT: ssh.fingerprint
 					}
 				}
+				if host != _|_ && ssh == _|_ {
+					DOCKER_HOST: host
+				}
 			}
 			"mount": {
 				if ssh != _|_ {