Change flag name to more discoverable TAKE_FILE_OWNERSHIP

Addresses
#125 (comment)

Author: dliappis

build/elasticsearch/bin/docker-entrypoint.sh

@@ -96,7 +96,7 @@ fi
 
 if [[ "$(id -u)" == "0" ]]; then
     # If requested and running as root, mutate the ownership of bind-mounts
-    if [[ -n "$MUTATE_MOUNTS" ]]; then
+    if [[ -n "$TAKE_FILE_OWNERSHIP" ]]; then
         chown -R 1000:0 /usr/share/elasticsearch/{data,logs}
     fi
 fi