dagger spec: fix Artifact definition

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

stdlib/dagger/dagger.cue

@@ -6,10 +6,11 @@ import (
 
 // An artifact such as source code checkout, container image, binary archive...
 // May be passed as user input, or computed by a buildkit pipeline
-#Artifact: #compute: [...llb.#Op]
-
-// deprecated, use #Artifact instead.
-#Dir: #Artifact
+#Artifact: {
+	#compute: [...llb.#Op]
+	_
+	...
+}
 
 // Secret value
 // FIXME: currently aliased as a string to mark secrets

stdlib/docker/docker.cue

@@ -9,7 +9,7 @@ import (
 
 // Build a docker container image
 #Build: {
-	source: dagger.#Dir
+	source: dagger.#Artifact
 
 	image: #compute: [
 		llb.#DockerBuild & {context: source},