support FROM...AS in dockerfile

extensions/docker/syntaxes/docker.tmLanguage.json

@@ -4,6 +4,14 @@
 	],
 	"name": "Dockerfile",
 	"patterns": [
+		{
+			"captures": {
+				"1": {
+					"name": "keyword.other.dockerfile"
+				}
+			},
+			"match": "\\s*(?:(FROM|AS))\\s"
+		},
 		{
 			"captures": {
 				"1": {