This can be done using docker compose. You will be able to access the files inside container by mounting them to external host directory. The files uploaded to wordpress are stored in wp-content/uploads
and you can mount this using this sample docker-compose file:
version: '3'
services:
mysql-database:
image: mysql
container_name: mysql-database
volumes:
- ./mysql/docker-entrypoint-initdb.d/:/docker-entrypoint-initdb.d
ports:
- 3306:3306
wordpress-sites:
image: wordpress:custom
build:
context: ./wordpress
dockerfile: wordpress.dockerfile
container_name: wordpress-sites
links:
- mysql-database
depends_on:
- mysql-database
ports:
- 8080:80
volumes:
- ./wordpress/wp-content:/var/www/html/wp-content
The documentation regarding volumes should be able to help you.