Fix Alpine missing /var/lib/postgresql/data mounts

[tianon]

This adds a mountinfo fallback that mimics coreutils.

• PostgreSQL 18: OLD_DATABASES detection fails to find data in /var/lib/postgresql/data/18/docker/ causing silent data loss on container recreation #1400 (comment)

Alternatively, we could just install coreutils and get the "real" mountpoint command.

Closes #1400

[tianon]

Before:

$ docker run -it --rm --pull=always -v /var/lib/postgresql/data -e POSTGRES_PASSWORD=example postgres:18-alpine
...
2026-04-21 18:04:38.822 UTC [1] LOG:  starting PostgreSQL 18.3 on x86_64-pc-linux-musl, compiled by gcc (Alpine 15.2.0) 15.2.0, 64-bit
2026-04-21 18:04:38.822 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2026-04-21 18:04:38.822 UTC [1] LOG:  listening on IPv6 address "::", port 5432
2026-04-21 18:04:38.825 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"

After:

$ docker run -it --rm --pull=always -v /var/lib/postgresql/data -e POSTGRES_PASSWORD=example --mount type=bind,src="$PWD/docker-entrypoint.sh",dst=/usr/local/bin/docker-entrypoint.sh,ro postgres:18-alpine
18-alpine: Pulling from library/postgres
Digest: sha256:52098013b4b64a746626437d38afc03cabff6cdeb4d3d92e2342aa95f0ce56ea
Status: Image is up to date for postgres:18-alpine
Error: in 18+, these Docker images are configured to store database data in a
       format which is compatible with "pg_ctlcluster" (specifically, using
       major-version-specific directory names).  This better reflects how
       PostgreSQL itself works, and how upgrades are to be performed.

       See also https://github.com/docker-library/postgres/pull/1259

       Counter to that, there appears to be PostgreSQL data in:
         /var/lib/postgresql/data (unused mount/volume)

       This is usually the result of upgrading the Docker image without
       upgrading the underlying database using "pg_upgrade" (which requires both
       versions).

       The suggested container configuration for 18+ is to place a single mount
       at /var/lib/postgresql which will then place PostgreSQL data in a
       subdirectory, allowing usage of "pg_upgrade --link" without mount point
       boundary issues.

       See https://github.com/docker-library/postgres/issues/37 for a (long)
       discussion around this process, and suggestions for how to do so.

[autorejecttop]

Finally, haha

I was crazy trying to find out why all my data was gone when using postgres:18.3-alpine3.23

I thank you in behalf of the future devs that may stumble upon this silent nightmare

[yosifkit]

Alternatively, we could just install coreutils and get the "real" mountpoint command.

I don't think it is worth the extra space and this awk is simple enough, so LGTM