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>

environment/pipeline.go

@@ -468,6 +468,11 @@ func (p *Pipeline) mount(ctx context.Context, dest string, mnt *compiler.Value)
 				llb.Scratch(),
 				llb.Tmpfs(),
 			), nil
+		case "docker.sock":
+			return llb.AddSSHSocket(
+				llb.SSHID(solver.DockerSocketID),
+				llb.SSHSocketTarget(dest),
+			), nil
 		default:
 			return nil, fmt.Errorf("invalid mount source: %q", s)
 		}