I\'d like to use drush. It needs to run in the drupal container. There\'s also a drush docker repo. But I have no clue how to make it available whithin the drupal container. It\
I think you have two options, you could run the drush/drush
container separately and use a shared hold volume so it has access to the files that drupal is going to use.
Or you could build a new image that contains both drush and drupal. See https://docs.docker.com/engine/userguide/containers/dockerimages/
You can probably use the Dockerfile for the drush image and the drupal image as guidance.
Try something like https://github.com/wodby/docker4drupal. For me is very useful and flexible thing to develop Drupal with Docker.