docker compose override a ports property instead of merging it

This behaviour is documented at https://docs.docker.com/compose/extends/#adding-and-overriding-configuration

For the multi-value options ports, expose, external_links, dns, dns_search, and tmpfs, Compose concatenates both sets of values

Since the ports will be the concatenation of the ports in all your compose files, I would suggest creating a new docker-compose.dev.yml file which contains your development port mappings, removing them from the base docker-compose.yml file.

As Nikson says, you can name this docker-compose.override.yml to apply your development configuration automatically without chaining the docker-compose files. docker-compose.override.yml will not be applied if you manually specify another override file (e.g. docker-compose -f docker-compose.yml -f docker-compose.prod.yml)