Fix etherpad plugins installation

isard-apps-etherpad           | [2022-01-15 05:04:23.738] [ERROR] runCmd|npm - npm WARN read-shrinkwrap This version of npm is compatible with lockfileVersion@1, but package-lock.json was generated for lockfileVersion@2. I'll try to do my best with it!

docker/etherpad/entrypoint.sh

@@ -92,7 +92,7 @@ if [ $ETHERPAD_PLUGINS ]; then
 	IFS=',' read -r -a PLUGIN_LIST <<< "$ETHERPAD_PLUGINS"
 	for PLUGIN in "${PLUGIN_LIST[@]}"
 	do
-	    npm install ${PLUGIN}
+	    ./src/node_modules/.bin/npm install ${PLUGIN}
 	done
 fi