Update highlights

queries/highlights.scm

@@ -1,4 +1,4 @@
-[ 
+[
 	"FROM"
 	"AS"
 	"RUN"
@@ -17,6 +17,8 @@
 	"STOPSIGNAL"
 	"HEALTHCHECK"
 	"SHELL"
+	"MAINTAINER"
+	"CROSS_BUILD"
 ] @keyword
 
 [
@@ -25,3 +27,25 @@
 ] @operator
 
 (comment) @comment
+
+
+(image_spec
+	(image_tag
+		":" @punctuation.special)
+	(image_digest
+		"@" @punctuation.special))
+
+(double_quoted_string) @string
+
+(expansion
+  [
+	"$"
+	"{"
+	"}"
+  ] @punctuation.special
+) @none
+
+((variable) @constant
+ (#match? @constant "^[A-Z][A-Z_0-9]*$"))
+
+