What is Docker Compose override and multiple Compose files?

Docker Compose supports multiple configuration files that are merged together, enabling environment-specific configurations without duplicating the base configuration. Override files: docker-compose.override.yml is automatically merged with docker-compose.yml when you run docker compose up without specifying files. This is typically used for development-specific overrides (volume mounts for live code reloading, debug ports, additional env vars). Multiple -f files: docker compose -f docker-compose.yml -f docker-compose.prod.yml up -d — merges the files in order. Later files override earlier ones for scalar values; lists (volumes, ports, environment) are merged/appended. Common pattern: docker-compose.yml — base service definitions; docker-compose.override.yml — development overrides (automatically loaded); docker-compose.prod.yml — production overrides (explicitly specified); docker-compose.test.yml — test environment configuration. Merging rules: string values are replaced by the later file; lists are concatenated. Create a merged output: docker compose -f docker-compose.yml -f docker-compose.prod.yml config — outputs the merged YAML without running anything. Use COMPOSE_FILE env var to set default file list.