Added ARG instruction and aligned with src repo

Docker-DCO-1.1-Signed-off-by: Asbjorn Enge <asbjorn@hanafjedle.net> (github: asbjornenge)

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

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
 	<key>fileTypes</key>
@@ -25,7 +25,7 @@
 				</dict>
 			</dict>
 			<key>match</key>
-			<string>^\s*(?:(ONBUILD)\s+)?(FROM|MAINTAINER|RUN|EXPOSE|ENV|ADD|VOLUME|USER|WORKDIR|COPY|LABEL|STOPSIGNAL)\s</string>
+			<string>^\s*(?:(ONBUILD)\s+)?(FROM|MAINTAINER|RUN|EXPOSE|ENV|ADD|VOLUME|USER|WORKDIR|COPY|LABEL|STOPSIGNAL|ARG)\s</string>
 		</dict>
 		<dict>
 			<key>captures</key>