Add an offline page to display when Discourse is rebuilding or starting up

[fefrei (Felix Freiberger)]

Note that this topic has significant overlap with that one:

Run other websites on the same machine as Discourse

@pfaffman edited this heavily 2022.02.24. Blame me if it’s broken. If you want to run other websites on the same machine as Discourse, you need to set up an extra NGINX or HAProxy proxy in front of the Docker container. NOTE: This is for advanced admins This guide assumes you already have Discourse working - if you don’t, it may be hard to tell whether or not the configuration is working. You cannot use ./discourse-setup to set up Discourse if another server is using port 80 or 443. You will …

That topic has received significant updating lately (that increased the overlap even more). Maybe it makes sense to merge the offline-page part from here to there as a note (since it’s easy to add if you already run a separate Nginx instance), then mark this one as deprecated (linking to the alternatives)?

Your suggested HAProxy topic would still make sense in addition, as the default way to go for people not wanting to install a front Nginx for other reasons.