For #19: added PVP env var options

* also adding a marker env var for MCCY to reference

minecraft-server/Dockerfile

@@ -25,9 +25,10 @@ WORKDIR /data
 
 CMD [ "/start" ]
 
-ENV UID 1000
+# Special marker ENV used by MCCY management tool
+ENV MC_IMAGE=YES
+
+ENV UID=1000
 ENV MOTD A Minecraft Server Powered by Docker
-ENV LEVEL world
 ENV JVM_OPTS -Xmx1024M -Xms1024M
-ENV TYPE VANILLA
-ENV VERSION LATEST
+ENV TYPE=VANILLA VERSION=LATEST LEVEL=world PVP=true