@CryPt00n
Thanks for the detailed answer !
First, I was referring to adminer not to use it to backup/migrate, but to point out that database and credentials are valid, as it can be accessed by adminer.
I tried your method, but when trying to backup database in source container, i get this error :
/ # pg_dumpall -U nextcloud -s > /var/lib/postgresql/data/bak.sql
pg_dumpall: error: query failed: ERROR: permission denied for table pg_authid
pg_dumpall: error: query was: SELECT oid, rolname, rolsuper, rolinherit, rolcreaterole, rolcreatedb, rolcanlogin, rolconnlimit, rolpassword, rolvaliduntil, rolreplication, rolbypassrls, pg_catalog.shobj_description(oid, 'pg_authid') as rolcomment, rolname = current_user AS is_current_user FROM pg_authid WHERE rolname !~ '^pg_' ORDER BY 2
Nextcloud db user is really "nextcloud", no error on that, like I said, database is accessed by the same user by adminer with no problem, and I can see all entries.
I don't know what I did wrong in my setup to end up with this result