In 12.0-alpine, passing the --version argument to docker run worked. This no longer works the same way in 12.1-alpine. I think the arguments are also being passed into pg_ctl in the startup script instead of just postgres which probably won't work in both places with the same arguments.

docker run --rm postgres:12.0-alpine --version

...
PostgreSQL init process complete; ready for start up.

postgres (PostgreSQL) 12.0

docker run --rm postgres:12.1-alpine --version

...
Success. You can now start the database server using:

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

waiting for server to start....2019-11-19 08:35:57.650 GMT [35] FATAL:  --version requires a value
 stopped waiting
pg_ctl: could not start server
Examine the log output.