[pica-etherpad-db] Correct permissions inside the container

pica-etherpad-db/Dockerfile

 FROM mysql:8
 
 COPY config/limit_binary_log.cnf /etc/mysql/conf.d/limit_binary_log.cnf
-#
+
+# By default, COPY uses permissions `-rw-rw-rw-` for the file inside
+# the container. But mysql refuses to use config files that are world-
+# writable, so we have to change those permissions:
+RUN chmod 644 /etc/mysql/conf.d/limit_binary_log.cnf