pkg: docker: add the command exit code

Signed-off-by: Sam Alba <samalba@users.noreply.github.com>

pkg/universe.dagger.io/docker/run.cue

@@ -160,4 +160,7 @@ import (
 			user: input.config.user
 		}
 	}
+
+	// Command exit code
+	exit: _exec.exit
 }