docker-compose: Sharing container between multiple projects by using the same container_name

You can simply avoid redefining the mysql in one of the two docker-compose.yml files and connect mysql and the other containers to the same network.

In order to do so, create a network:

docker network create shared

Assign your network to your mysql container:

version: '3'
services:
  mysql:
    ...
    networks:
    - shared
networks:
  shared: 
    external:
      name: shared

For any other container that has need to access mysql, just add the same network definition as above:

version: '3'
services:
  app1:
    ...
    networks:
    - shared
  app2:
    ...
    networks:
    - shared
  ...
networks:
  shared: 
    external:
      name: shared