Having two files

docker-compose.yml with:

version: '2.1'
services:
  api:
    volumes:
      - ~/src/projects/p1:/app
  db:
    volumes:
      - ~/docker/projects/mysql5:/var/lib/mysql

and

docker-compose.override.yml with:

version: '2.1'
services:
  api:
    volumes:
      - /Users/me/projects/p1:/app
  db:
    volumes:
      - /Users/me/projects/p1/.docker/mysql5:/var/lib/mysql

it does not merge the volumes, instead of having /Users/me/projects/p1:/app, I see /Users/me/src/projects/p1:/app as if there was an expansion of the home dir from the first docker-compose.yml which is not the case when using docker/docker-compose