refactor: allow usage of the same variable to disable docker pulls

install

@@ -5,8 +5,8 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 plugin-install() {
   pull-docker-image() {
     declare IMAGE="$1"
-    if [[ "$MONGO_DISABLE_PULL" == "true" ]]; then
-      echo " !     MONGO_DISABLE_PULL environment variable detected. Not running pull command." 1>&2
+    if [[ "$PLUGIN_DISABLE_PULL" == "true" ]]; then
+      echo " !     ${PLUGIN_DISABLE_PULL_VARIABLE} environment variable detected. Not running pull command." 1>&2
       echo " !        docker pull ${IMAGE}" 1>&2
       return
     fi