[EtherpadDB] Forgot to fix permissions of new config file

pica-etherpad-db/Dockerfile

@@ -7,6 +7,7 @@ COPY config/myisam_perf.cnf /etc/mysql/conf.d/myisam_perf.cnf
 # 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/binary_log.cnf
+RUN chmod 644 /etc/mysql/conf.d/myisam_perf.cnf
 
 # Remove unnecessary tools
 # This removes the following packages: