$ docker run --rm --name some-postgres -d postgres
cb2ddbb0f4f715077ebc1bfc2dc7151e5a6d07cd374c28be1db6d6ad77b9b16a
$ docker run -it --rm --link some-postgres:postgres postgres psql -h postgres -U postgres
psql (10.4 (Debian 10.4-2.pgdg90+1))
Type "help" for help.
postgres=#
Você esqueceu o -U postgres