expose:

https://docs.docker.com/compose/compose-file/compose-file-v3/#expose

Expose ports without publishing them to the host machine - they'll only be accessible to linked services. Only the internal port can be specified.

• EXPOSE
• ports:

See also

• docker-compose.yml: services:, volumes:, configs:, secrets:, networks:, driver:, depends_on:, extends:
• docker-compose, docker-compose.yml: [services | volumes | networks | up | down | restart | logs | create | changelog | version | secrets | config | -v | -h | exec], docker-compose pull, Environment:, Docker-compose.yml examples, .env, release notes, expose:, docker-compose --env-file, services.deploy.resources