The docker-compose.override.yml should not be ignored by default (#5838)

While this file can be used for local overrides, that is not its
only intended usage. E.g. a common setup would be like this:

- *docker-compose.yml*: services shared across builds
- *docker-compose.override.yml*: services only used in development
- *docker-compose.production.yml*: configuration needed for building production images.

Now for regular development you just need to run `docker-compose up --build` and only
in you CI you would build and run for production by explicitly naming the yml files.

TL;DR: Excluding docker-compose.override.yml seems to be a personal preference of
someone and they should do that in their global .gitignore if the are so inclined.

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