feat: implement GitHub Actions workflow for building

[francoism90]

This pull request introduces significant improvements to the container build and deployment pipeline, focusing on multi-platform image builds, enhanced Podman quadlet templates, and improved runtime user handling. The changes streamline the CI/CD workflow, standardize image references, and ensure containers run with correct user permissions across environments.

CI/CD Pipeline Enhancements:

• Added a comprehensive GitHub Actions workflow in .github/workflows/build.yml to build, tag, and push multi-platform images (linux/amd64, linux/arm64) using Podman and Buildah, with caching and manifest merging for efficient and reliable releases.

Podman Quadlet Template Updates:

• Updated all frankenphp-octane quadlet stubs (app, horizon, inertia-ssr, reverb, schedule) to use a configurable {{image}} placeholder (defaulting to a registry image), switch AutoUpdate to registry, and set PUID/PGID environment variables for user mapping. [1] [2] [3] [4] [5]
• Added a new substitutions config in config/podman.php to allow custom placeholder values (e.g., {{image}}) in quadlet templates.

Container Runtime Improvements:

• Modified the Containerfile to install gosu and ensure the container runs as root by default, so entrypoint logic can safely remap the docker user/group to the host's PUID/PGID before dropping privileges. [1] [2]
• Enhanced entrypoint.sh to dynamically renumber the docker user/group at runtime, ensuring correct file ownership and permission handling regardless of the host user's UID/GID.

Platform-Specific Dependency Handling:

• Updated the Containerfile to only install certain video driver packages (libmfx-gen1.2, libvpl2, intel-media-va-driver-non-free) on amd64 builds, reducing unnecessary dependencies on other platforms.