Volume bind mount path changed with postgres v18

[tobi-dub]

When setting up a complete new pgbackweb stack with your provided compose example, I get the following error:

$ docker compose up 
[+] Running 3/3
 ✔ Network pgbackweb_default  Created                                                                                                                                          0.1s 
 ✔ Container postgres         Created                                                                                                                                          0.1s 
 ✔ Container pgbackweb        Created                                                                                                                                          0.0s 
Attaching to pgbackweb, postgres
postgres  | Error: in 18+, these Docker images are configured to store database data in a
postgres  |        format which is compatible with "pg_ctlcluster" (specifically, using
postgres  |        major-version-specific directory names).  This better reflects how
postgres  |        PostgreSQL itself works, and how upgrades are to be performed.
postgres  | 
postgres  |        See also https://github.com/docker-library/postgres/pull/1259
postgres  | 
postgres  |        Counter to that, there appears to be PostgreSQL data in:
postgres  |          /var/lib/postgresql/data (unused mount/volume)
postgres  | 
postgres  |        This is usually the result of upgrading the Docker image without
postgres  |        upgrading the underlying database using "pg_upgrade" (which requires both
postgres  |        versions).
postgres  | 
postgres  |        The suggested container configuration for 18+ is to place a single mount
postgres  |        at /var/lib/postgresql which will then place PostgreSQL data in a
postgres  |        subdirectory, allowing usage of "pg_upgrade --link" without mount point
postgres  |        boundary issues.
postgres  | 
postgres  |        See https://github.com/docker-library/postgres/issues/37 for a (long)
postgres  |        discussion around this process, and suggestions for how to do so.
postgres exited with code 1
dependency failed to start: container postgres exited (1)

In your example compose file with postgres v18, the volume bind mount path in the container is wrong. With postgres v18, it changed from /var/lib/postgresql/data to /var/lib/postgresql.

After changing it, the database starts successfully.

$ docker compose up
[...]
pgbackweb-db  | 2025-11-14 20:54:42.081 UTC [1] LOG:  database system is ready to accept connections