Update scenario documentation to use docker-compose.override.yml

[YetAnotherGerrit]

As a system administrator, I want the documentation on how to enable scenarios to describe using a docker-compose.override.yml file with the include keyword, so that I can run scenarios with the plain docker compose up command instead of remembering scenario-specific -f flag combinations.

Acceptance criteria

• The docs.zammad.org installation/docker guide no longer instructs admins to run docker compose -f docker-compose.yml -f scenarios/{scenario}.yml up -d to enable a scenario.
• The guide instead documents creating a docker-compose.override.yml file that references one or more scenario files via include, for example:

  • include:
      - scenarios/first.yml
      - scenarios/second.yml

[ralf401]

See also zammad/zammad-docker-compose#618