Selfhosted

Docker automatically upgrades if you tell it to by specifying "latest" or not specifying a version number. But it only upgrades if you issue the pull command or the compose up command. There are ways to start without a pull like using start or restart. So yes, there was warning and something you did actively told it to upgrade.

And it's really bad practice to update any software without testing, especially between breaking/major version numbers.

Finally, it's not uncommon for a platform to release its update and then the plugins or addons to follow. Especially with major updates that require lots of testing before release. This allows plugin/add-on makers to fully test their software with the release version of the platform rather than all of the plugin makers having to wait for one that may be lagging behind.

Why did you do automatic updates without testing? That is the real issue.

Honestly your IT department sounds like it could use some help

Wait, you update productions systems without running a staging environment? Or even checking the update notes and your installed apps? Also no backups? What kind of business are you running over there?

Backups and rollbacks should be your next endeavor.

Specify a Version Tag in docker compose and update nextcloud deliberately through the webapp, that way it doesn't update automatically on a pull

The docker image automatically updated the install to nextcloud 30, but the forms app requires nextcloud 29 or lower.

Lol. Do not blame others for your incompetence. If you have automatically updates enabled then that is your fault when it breaks things. Just pin the major version with a tag like nextcloud:29 or something. Upgrading major versions automatically in production is a terrible decision.

take a VM snapshot, upgrade the app, validate it still works as intended, if not, revert from snapshot

There was a recent related discussion on Hacker News and the top comment discusses why this sort of solution is not likely to be the best fit for smaller organizations. In short, doing it well requires time and effort from someone technically sophisticated, who must do more than the bare minimum for good results, as you just learned.

Even then, it's likely to be less reliable than solutions hosted by big corporations and when there's a problem, it's your problem. I don't want to discourage you, but understand what you're committing to and make sure you have adequate buy-in in your organization.

The forms app is useless. It's basically for surveys. I can't see how you'd use it for signups.

Never upgrade to the latest and greatest of ... anything really, especially in production. Let others test it first, or as suggested already, have a staging environment where you test the upgrade first. I guess you can still downgrade nextcloud though, especially if you have a backup.

Are you using the AIO image? I don't know how well that works, but yeah, I absolutely hate automatic updates like that. I tried it once and I decided to use the plain "official but not supported" docker image instead, where I manage things myself. Never had an issue, and I can control which version I'm running, I can backup to wherever I want, using whichever system I want, etc.