Update etherpad to 1.8.6

Dockerfile

-FROM node:alpine 
+FROM node:alpine
 ARG NODE_ENV=production
 RUN mkdir /app && apk update && apk add curl gettext nginx && mkdir /run/nginx
-COPY ./etherpad-lite-1.8.4.tar.gz /app
+COPY ./etherpad-lite-1.8.6.tar.gz /app
 WORKDIR /app
-RUN tar zxvf etherpad-lite-1.8.4.tar.gz --strip 1 -C /app && \
-  rm /app/etherpad-lite-1.8.4.tar.gz
+RUN tar zxvf etherpad-lite-1.8.6.tar.gz --strip 1 -C /app && \
+  rm /app/etherpad-lite-1.8.6.tar.gz
 RUN /app/bin/installDeps.sh
 COPY ./settings.json.tmpl /app/settings.json.tmpl
 COPY ./docker-entrypoint.sh /docker-entrypoint.sh