feat: roundcube dev stack

[kulmann]

Description

This PR adds a roundcube service to our dev stack so that https://github.com/mschneider82/opencloud-webmail can be tested easily. Download and build the webmail app and place it in dev/docker/apps. Add a config.json file inside that folder with the following content:

{
  "config": {
    "instances": [
      {
        "id": "default",
        "label": "Dev Mail (Stalwart)",
        "roundcubeUrl": "https://host.docker.internal:9443",
        "sharedSecret": "dev-shared-secret"
      }
    ]
  }
}

You will find an Mail accounts nav item in the user preferences. Add an account and use ldap credentials (e.g. lynn:demo). The email addresses of our demo users are their usernames and @example.org.

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)