docker socket forwarding support

- This PR adds a new mount type: `docker.sock` (in addition to `cache`
  and `tmp`)
- It's then able to mount the LOCAL (as in, from the machine running
  dagger) docker socket inside the container by pretending to be an SSH
  Agent (hijacking the SSH agent forwarding support of buildkit)

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

stdlib/dagger/op/op.cue

@@ -52,7 +52,7 @@ package op
 	// `true` means also ignoring the mount cache volumes
 	always?: true | *false
 	dir:     string | *"/"
-	mount: [string]: "tmpfs" | "cache" | {from: _, path: string | *"/"} | {secret: _}
+	mount: [string]: "tmpfs" | "cache" | "docker.sock" | {from: _, path: string | *"/"} | {secret: _}
 	// Map of hostnames to ip
 	hosts?: [string]: string
 	// User to exec with (if left empty, will default to the set user in the image)