This fixes the problem of extracting variables from a list of maps. Currently, variables from such an entity would be ignored. A simple way of reproduce this is to execute the following command:

docker compose -f docker-compose.volumes.yml config --variables

The content of docker-compose.volumes.yml is:

services:
    postgres:
        environment:
            POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-postgres}
            POSTGRES_USER: postgres
        image: postgres
        volumes:
            - ${CERT_FILE}:/certs/server.crt:ro

The output will be:

NAME                REQUIRED            DEFAULT VALUE       ALTERNATE VALUE
POSTGRES_PASSWORD   false               postgres            

CERT_FILE is missing. Internally, the volumes will be represented as volumes:[map[read_only:true source:${CERT_FILE} target:/certs/server.crt type:volume volume:map[]]] .