This client is compatible with the official postgresql image, which means that it uses the environment variables provided by the image.
docker run -e POSTGRES_USER=user -e POSTGRES_PASSWORD=pwd -d --name=pg_container postgres:9.5
docker run --rm --link pg_container:db kalumkalac/postgresql-client:9.5 psql -c 'create database foo'
docker run --rm --link pg_container:db -v /tmp/foo:/dump kalumkalac/postgresql-client:9.5 pg_dump foo -f /dump/dump_foo.sql