Support FROM...AS syntax for multi stage build dockerfiles

Signed-off-by: chrisdias <cdias@microsoft.com>

contrib/syntax/textmate/Docker.tmbundle/Syntaxes/Dockerfile.tmLanguage

@@ -10,6 +10,23 @@
 	<string>Dockerfile</string>
 	<key>patterns</key>
 	<array>
+		<dict>
+			<key>captures</key>
+			<dict>
+				<key>1</key>
+				<dict>
+					<key>name</key>
+					<string>keyword.other.special-method.dockerfile</string>
+				</dict>
+				<key>2</key>
+				<dict>
+					<key>name</key>
+					<string>keyword.other.special-method.dockerfile</string>
+				</dict>
+			</dict>
+			<key>match</key>
+			<string>^\s*\b(FROM)\b.*?\b(AS)\b</string>
+		</dict>
 		<dict>
 			<key>captures</key>
 			<dict>