Set RUN_MODE prod by default (#13765)

I think it's a bad default to have "dev" as the default run mode which
enables debugging and now also disables HTTP caching. It's better to
just default to a value suitable for general deployments.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

docker/root/etc/s6/gitea/setup

@@ -25,7 +25,7 @@ if [ ! -f ${GITEA_CUSTOM}/conf/app.ini ]; then
 
     # Substitude the environment variables in the template
     APP_NAME=${APP_NAME:-"Gitea: Git with a cup of tea"} \
-    RUN_MODE=${RUN_MODE:-"dev"} \
+    RUN_MODE=${RUN_MODE:-"prod"} \
     DOMAIN=${DOMAIN:-"localhost"} \
     SSH_DOMAIN=${SSH_DOMAIN:-"localhost"} \
     HTTP_PORT=${HTTP_PORT:-"3000"} \