Documentation on updating Ghost

With the release of Ghost v5 we have spent quite a lot of time carefully crafting update docs

https://ghost.org/docs/update

However, they are for the recommended install method which is Ghost CLI. We use the CLI to do a lot of the legwork and prevent issues with updates.

With docker, I'm not sure there's any way we can do anything to prevent people updating incorrectly, but it seems there should probably be detailed update docs?

The critical details being:

1. mysql8 is now the only supported database in production.
2. updates across majors are only guaranteed to work if you update to the latest version in your major first.

To clarify that last point, if you're on 3.25.9, you need to update Ghost to 3.42.9 - including starting the server to trigger the migrations - before updating to v5.0.0.

Note: you don't have to cycle through every major - you don't have to upgrade to v4 first.

There's absolutely no way to guarantee you can update from any version to any version without data loss or corruption as there can always be bugs that are fixed in later versions 🤷‍♀️

Start with the existing update guidance at https://ghost.org/docs/update and inspect the docker-library-ghost repository for its current documentation and update instructions. Document Docker-specific upgrades, including MySQL 8 production support and updating to the latest release in the current major with the server started before crossing majors. Done means the guidance also explains that intermediate major versions are not required.