Fatal error during migration 16-to-17

[tamimology]

I have executed the following to migrate from v16 to v17

docker run --rm -v /home/USER/docker/db-16:/var/lib/postgresql/16/data -v /home/USER/docker/db-17:/var/lib/postgresql/17/data tianon/postgres-upgrade:16-to-17

The output result was

The files belonging to this database system will be owned by user "postgres".
This user must also own the server process.

The database cluster will be initialized with locale "en_US.utf8".
The default database encoding has accordingly been set to "UTF8".
The default text search configuration will be set to "english".

Data page checksums are disabled.

fixing permissions on existing directory /var/lib/postgresql/17/data ... ok
creating subdirectories ... ok
selecting dynamic shared memory implementation ... posix
selecting default "max_connections" ... 100
selecting default "shared_buffers" ... 128MB
selecting default time zone ... Etc/UTC
creating configuration files ... ok
running bootstrap script ... ok
performing post-bootstrap initialization ... ok
syncing data to disk ... ok

Success. You can now start the database server using:

    pg_ctl -D /var/lib/postgresql/17/data -l logfile start

initdb: warning: enabling "trust" authentication for local connections
initdb: hint: You can change this by editing pg_hba.conf or using the option -A, or --auth-local and --auth-host, the next time you run initdb.
Performing Consistency Checks
-----------------------------
Checking cluster versions                                     ok

connection to server on socket "/var/lib/postgresql/.s.PGSQL.50432" failed: FATAL:  role "postgres" does not exist

could not connect to source postmaster started with the command:
"/usr/lib/postgresql/16/bin/pg_ctl" -w -l "/var/lib/postgresql/17/data/pg_upgrade_output.d/20240930T133216.336/log/pg_upgrade_server.log" -D "/var/lib/postgresql/16/data" -o "-p 50432 -b  -c listen_addresses='' -c unix_socket_permissions=0700 -c unix_socket_directories='/var/lib/postgresql'" start
Failure, exiting

Not sure what the issue is, but I can see that the port inside the script is set to 50432 where my understanding is that Postgres uses port 5432. What do you reckon the issue is?