[Etherpad] Create pad backups in Dockerfile

This will fix permissions issues : a Docker Volume mounted on a non-existing directory will be owned by root, but we run Etherpad as etherpad. As such we have no way of writing in the mounted directory

pica-etherpad/Dockerfile

@@ -66,6 +66,9 @@ RUN apt-get update && \
     chown -R etherpad /opt/etherpad-lite && \
     rm -rf /var/lib/apt/lists/*
 
+RUN mkdir /opt/etherpad-lite/deleted_pads && \
+    chown -R etherpad:etherpad /opt/etherpad-lite/deleted_pads
+
 EXPOSE 8080
 HEALTHCHECK --interval=20s --timeout=3s CMD curl --fail http://localhost:8080 || exit 1
 WORKDIR /opt/etherpad-lite/