This allows for a more graceful error message for the case of users who need to update their volume path.
I tested this with 17 and `--env PGDATA=/var/lib/postgresql/17/docker` (to make sure it still works fine and doesn't complain about the extra volume provided by Docker/`VOLUME`) and 18 with `-v /var/lib/postgresql/data` (where it appropriately errors and explains to me what I should be doing instead).
I also did a reflow on some text in the error message that's designed to fit in 80 columns but was taking 81. 😂
This is a bit complicated to use in Docker (because `io_uring` has a long and storied security story), but it is a new feature of 18+ that we should probably explicitly support for users who want to jump through the hoops to use it despite the security risk.
Additionally, this re-alphabetizes our `--with-xxx` flags.
This is a pretty large breaking change, which is why this only makes the change in 18+ (which is currently in pre-release stages, and not due for GA until September, and pre-release `PGDATA` directories are officially *not supported* on the GA release anyhow).
Concretely, this changes `PGDATA` to `/var/lib/postgresql/MAJOR/docker`, which matches the pre-existing convention/standard of the `pg_ctlcluster`/`postgresql-common` set of commands, and frankly is what we should've done to begin with, in a classic case of Chesterton's Fence (https://en.wikipedia.org/wiki/Wikipedia:Chesterton%27s_fence).
This also changes the `VOLUME` to `/var/lib/postgresql`, which should be more reasonable, and make the upgrade constraints more obvious.
For any users who have been testing the pre-releases, the simplest way to keep your existing data directory is going to be to add `PGDATA=/var/lib/postgresql/data` as an environment variable on your container or adjust your bind-mount from `/var/lib/postgresql/data` to `/var/lib/postgresql/18/docker`, but the *best* way is going to be to refactor your host directory such that your data lives at `18/docker` inside and you can then mount directly to `/var/lib/postgresql` (possibly setting `PGDATA=/var/lib/postgresql/MAJOR/docker` as well, if you want to go overboard on being explicit).
Tianon Gravi