How do I run a sql file of inserts through docker run?

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

You can run a sql command file against a running postgres container via the one liner

cat ./query.sql | docker exec -i <container-name> psql -U <user> -d <database>

without having to copy the file from your host machine to the container.