.env
Used to set the values of undefined environment variables found in the default docker-compose.yml file directly.
custom.env
Used to add new or override existing environment variable values within the default.env file. (The default.env file is used to set environment variables within the running containers).
docker-compose-custom.yml
Used to add new or override existing entries within the default docker-compose.yml file. Note that the override structure must match the one present in docker-compose.yml.
# example of overriding the ssl certificate used by nginx
# services:
# nginx:
# volumes:
# - /path/to/certificate/file.crt:/etc/nginx/conf.d/ssl/certs/nginx_ssl_cert