I'm upgrading an existing dockerized postgres:9.6 to 10. There are plenty of good Q/As on DBA and SO that go the path of `pg_upgradecluster`, but it is not working for me.

When I start a vanilla pg:10 instance, whether I mount the old volume or not, `pg_lsclusters` is empty.

    root@server:# docker run -it -d --name pg10 postgres:10
    ...
    fixing permissions on existing directory /var/lib/postgresql/data ... ok
    creating subdirectories ... ok
    selecting default max_connections ... 100
    ...
    2018-07-15 05:11:52.569 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
    2018-07-15 05:11:52.635 UTC [68] LOG:  database system was shut down at 2018-07-15 05:11:52 UTC
    2018-07-15 05:11:52.651 UTC [1] LOG:  database system is ready to accept connections

Connections work:

    root@server:# docker exec -it pg10 psql -h localhost -U postgres postgres
    psql (10.4 (Debian 10.4-2.pgdg90+1))
    Type "help" for help.
    
    postgres=#

But clusters are not found:

    root@server:# docker exec -it pg10 su postgres -c pg_lsclusters
    Ver Cluster Port Status Owner Data directory Log file

(Nothing given.) If that will work, then I'm intending to mount the old db volume as another cluster (via steps listed in https://stackoverflow.com/a/47233300/3358272).

I don't understand enough about the db layout to know why the default postgresql image does not produce its default database as a cluster that can be seen with this utility. Help?