Adding Docker 1.12 instructions to its grammar

This simply updates the Docker grammar to include the `HEALTHCHECK` and `SHELL` commands that were added to Docker 1.12, and were added to the [grammar file](https://github.com/docker/docker/blob/4cb71f80823af345d063cf0ad657e73ce9caa75f/contrib/syntax/textmate/Docker.tmbundle/Syntaxes/Dockerfile.tmLanguage) which VS Code's copy is based on.

extensions/docker/syntaxes/Dockerfile.tmLanguage

@@ -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|ARG)\s</string>
+			<string>^\s*(?:(ONBUILD)\s+)?(ADD|ARG|CMD|COPY|ENTRYPOINT|ENV|EXPOSE|FROM|HEALTHCHECK|LABEL|MAINTAINER|RUN|SHELL|STOPSIGNAL|USER|VOLUME|WORKDIR)\s</string>
 		</dict>
 		<dict>
 			<key>captures</key>
@@ -140,4 +140,4 @@
 	<key>uuid</key>
 	<string>a39d8795-59d2-49af-aa00-fe74ee29576e</string>
 </dict>
-</plist>
+</plist>