chore: update compose-go spec to v1.20.0 with lagoon fixes

[shreddedbacon]

This updates the compose-go library to 1.20.0 with the additional fixes required for lagoon.

Unfortunately though, this resulted in additional cases where things people may have done in previous versions of docker compose worked, do not work anymore.

Example, previously people may have defined <<: *default-volumes and <<: *default-user like so, but under the latest compose-go spec, this is not supported.

  php:
    build:
      context: .
      dockerfile: .docker/Dockerfile.php
      args:
        CLI_IMAGE: *project
    <<: *default-volumes
    <<: *default-user
    environment:
      <<: *default-environment
    depends_on:
      - cli
    labels:
      lagoon.type: nginx-php-persistent
      lagoon.name: nginx-php
      lagoon.persistent: /app/docroot/sites/default/files/

closes #252

[shreddedbacon]

Leaving as draft until we make a call on when to fail builds for invalid docker-compose files or drop our fork

[shreddedbacon]

closing in favour of #304