Thanks JammyDodger! The info and link were very helpful. I’m not sure what constitutes a container restart - as far as I know my container is not restarting every ~1:30 which is as long as the RO mode stays enabled when using discourse enable_readonly i.e. Discourse.enable_readonly_mode(Discourse::READONLY_MODE_KEY)
I can confirm however that Discourse.enable_readonly_mode(Discourse::USER_READONLY_MODE_KEY) “sticks” indefinitely, which is what I was trying to achieve 