to execute commands against a running container use docker exec.
to copy a file (ex: dump.sql) into a container, use docker cp
So your approach might look something like this:
docker cp ./dump.sql pg_test:/docker-entrypoint-initdb.d/dump.sql
docker exec -u postgres pg_test psql postgres postgres -f docker-entrypoint-initdb.d/dump.sql
here it is in generic form:
docker cp ./localfile.sql containername:/container/path/file.sql
docker exec -u postgresuser containername psql dbname postgresuser -f /container/path/file.sql
And note that if you need to seed your database every time it is run, the folder /docker-entrypoint-initdb.d/ does have special significance, if you’re using the offical postgres image