Add support for writing client CAs when access-lists are updated

This commit adds the basic support necessary to produce the combined
client CA files when certificates are updated.

docker/rootfs/etc/s6-overlay/s6-rc.d/prepare/20-paths.sh

@@ -20,6 +20,7 @@ mkdir -p \
 	/data/custom_ssl \
 	/data/logs \
 	/data/access \
+	/data/clientca \
 	/data/nginx/default_host \
 	/data/nginx/default_www \
 	/data/nginx/proxy_host \