Use LABEL instead of MAINTAINER (envygeeks#162)

Per docker docs MAINTAINER appears to be deprecated in favor of a more general LABEL command: https://docs.docker.com/engine/reference/builder/#maintainer-deprecated

repos/jekyll/Dockerfile

@@ -1,5 +1,5 @@
 FROM envygeeks/alpine
-MAINTAINER Jekyll Core <[email protected]>
+LABEL maintainer "Jekyll Core <[email protected]>"
 COPY copy/ /
 ENV LANGUAGE=en_US
 ENV LANG=en_US.UTF-8