Abort if standby.signal is found

[ElDavoo]

I'm testing with the standby DBs before trying the main one and this happens:

db-delay-1  | Determining our own initdb arguments
db-delay-1  | ------------------------------------
db-delay-1  | 2025-01-14 12:13:35.535 UTC [36] LOG:  database system was shut down in recovery at 2025-01-14 12:11:34 UTC
db-delay-1  | 2025-01-14 12:13:35.535 UTC [36] FATAL:  standby mode is not supported by single-user servers
db-delay-1 exited with code 0

The script should handle the existence of standby.signal in some way.

[ElDavoo]

Apparently (well it's obvious now that I think about it), upgrading creates a new DB cluster, regenerating the system identifier, which needs to be the same between primary and standby.
So, this container should say something like "Hey, you can't do this, recreate the hot standby"
If you remove standby.signal, the migration works, but after that you'll get
FATAL: database system identifier differs between the primary and standby