Imagine you have a complex microservice. For local development, you need it connected to a message queue, a telemetry collector, and a database. But for your E2E testing, you need a slightly modified version (different env vars, an extra mock dependency, maybe a different port).
Most engineers solve this by maintaining two massive, almost-identical YAML files. It is a nightmare to sync changes.
Or they simply do this:
$ docker compose -f compose.yml -f e2e-compose.yml up --build -d
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