Merge pull request #6949 from nextcloud/jtr/docs-containers-mastercontainer-readme

docs: Add README for `mastercontainer`

Containers/mastercontainer/README.md

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+# Nextcloud All-in-One `mastercontainer`
+
+This folder contains the OCI/Docker container definition, along with associated resources and
+configuration files, for building the `mastercontainer` as part of the Nextcloud All-in-One
+project. This container hosts [the Nextcloud AIO interface](
+https://github.com/nextcloud/all-in-one/tree/main/php)[^app], and a dedicated PHP environment
+for it (which is completely independent of the Nextcloud Server).
+
+## Overview
+
+The mastercontainer acts as the central orchestration service for the deployment and management
+of all other containers in the Nextcloud All-in-One stack. It hosts:
+
+- A dedicated PHP SAPI/backend (php-fpm) for AIO itself (not Nextcloud Server)
+- An Apache service for accessing the AIO interface via a self-signed HTTPS VirtualHost on 8080/tcp
+- A Caddy reverse proxy service enabling HTTPS access to the AIO frontend on port 8443/tcp.
+  - Caddy will automatically issue a Let's Encrypt issued certificate if port 80 and 8443
+    is open/forwarded and a domain pointer is in place; then, simply open the Nextcloud AIO interface using the
+    domain (`https://your-domain-that-points-to-this-server.tld:8443`). The Let's Encrypt certificate request will
+    use an [ACME HTTP-01](https://letsencrypt.org/docs/challenge-types/#http-01-challenge) challenge.
+- Miscellaneous support services specific to AIO (backup management, health checks, etc.)
+
+## Key Responsibilities
+
+- Orchestrates the deployment and lifecycle of all Nextcloud service containers
+- Handles initial setup and container configuration
+- Coordinates image updates
+- Monitors general system health
+
+It triggers the initial installation and ensures the smooth operation of the Nextcloud
+All-in-One stack.
+
+## Contents
+
+- **Dockerfile**: Instructions for building the mastercontainer image.
+- **Entrypoint script**: The `start.sh` script is used for container initialization and runtime
+  configuration before starting supervisord.
+- [**Nextcloud All-in-One Controller App**](https://github.com/nextcloud/all-in-one/tree/main/php): The
+  core AIO orchestrator that handles configuration and settings for the containers.
+- **Supervisor**: The `supervisord.conf` file defines the long-running services hosted within
+  the container (php-fpm, cron, etc.)
+
+## Usage
+
+This container should be used as the trigger image when deploying the Nextcloud All-in-One
+stack in a Docker or other OCI-compliant container environment. For detailed deployment
+instructions, refer to the [project documentation](
+https://github.com/nextcloud/all-in-one).
+
+## Related Resources
+
+- [Main Repository](https://github.com/nextcloud/all-in-one)
+- [Documentation](https://github.com/nextcloud/all-in-one#readme)
+
+## Contributing
+
+Contributions are welcome! Please follow the Nextcloud project's guidelines and submit pull
+requests or issues via the main repository.
+
+## License
+
+This folder and its contents are licensed under the
+[GNU AGPLv3](https://www.gnu.org/licenses/agpl-3.0.html), in line with the rest of Nextcloud
+All-in-One.
+
+[^app]: The Nextcloud All-in-One interface allows users to install, configure, and
+manage their Nextcloud instance and related containers via a secure web interface and API.
+It automates and simplifies complex tasks such as container orchestration, backups, updates,
+and service management for users deploying Nextcloud in Docker environments.