How to inherit/merge volumes from yaml anchors in docker-compose?

Question

I want my docker-compose file to merge (reuse) volumes definitions, as follows:

x-defaults: &my-defaults:
  volumes:
    - /first:/volume
    - /second:/volume

services:  
  my-service1:
    <<: *my-defaults
    volumes:
      - /additional:/volume
 
  my-service2:
    <<: *my-defaults
    volumes:
      - /custom:/vol

Result: only the /additional:/volume is mapped.

Question: how can I achieve a real merge here?

Answer 1

Simple answer: it's not possible. Array lists are replaced on inheritance, thus when overriding, have to repeat all list statements.