[Docker] Split config dir creation/set permissions

If the config dir already exists, setting the mode (`-m 777`) doesn't
actually work as it should. This change splits the command into two
separate commands for directory creation and enabling the directory to
be writable by all.

Fixes #658

Dockerfile

@@ -22,7 +22,8 @@ RUN apk -U upgrade
 ARG DOCKER_USER=whoogle
 ARG DOCKER_USERID=927
 ARG config_dir=/config
-RUN mkdir -p -m 777 $config_dir
+RUN mkdir -p $config_dir
+RUN chmod a+w $config_dir
 VOLUME $config_dir
 
 ARG username=''