Docker Compose

docker-compose

By default, Compose reads two files, a docker-compose.yml and an optional docker-compose.override.yml file. By convention, the docker-compose.yml contains your base configuration. The override file, as its name implies, can contain configuration overrides for existing services or entirely new services.
- For single-value options like image, command or mem_limit, the new value replaces the old value.
- For the multi-value options ports, expose, external_links, dns, dns_search, and tmpfs, Compose concatenates both sets of values.
- In the case of environment, labels, volumes, and devices, Compose "merges" entries together with locally-defined values taking precedence. For environment and labels, the environment variable or label name determines which value is used.

https://docs.docker.com/compose/compose-file/
Top-level version property is defined by the specification for backward compatibility but is only informative.¹