You need to define docker volumes where you can add your custom modules. You can use a folder on your computer where your custom modules will be stored (e.g., odoo/addons/custom_module_1, odoo/addons/custom_module_2) and link this to the extra-addons folder in the Docker container using the -v flag in $ docker run (e.g., -v /odoo/addons:/mnt/extra-addons​) Alternatively you can use docker to create volumes: $ docker volume create extra-addons​​ and use this volume -v extra-addons:/mnt/extra-addons​. The content of this volume can usually be found at /var/lib/docker/volumes/​ or run $ docker volume inspect extra-addons​​ to find the mount point. If you run $ docker volume ls​​ you will get a list of all volumes that Docker uses. You can use the local path (e.g., odoo/addons/custom_module_1) in Pycharm to start creating your own custom module and it will automatically be linked to the addons path in the container.

In addition you need to add to add this path to the odoo.conf file: addons_path = /mnt/extra-addons​. This folder should also be stored in a docker volume. To add this to a volume you can use the same procedure as above: -v /odoo/config:/etc/odoo​. Also make sure to restart Odoo when making changes.

If you want to know more about how to deploy Odoo in docker containers you can also check out my blog post: 

Dock, Stack & Launch: Odoo deployment with Docker and Portainer

I hope this helps!