feat: add workflow to build ERPNext Docker image with ALYF custom apps

[MarcCon]

This PR introduces a GitHub workflow to automatically build a Docker image containing ERPNext along with ALYF custom apps.

What’s Included

apps.json:

• A configuration file that defines all apps and their branches to be included in the Docker Image.

build_image.yml:

• Builds a Docker image using frappe_docker as the base
• Uses a multi-platform build for amd64 and arm64
• Pushes the final image to a container registry

Notes:

The resulting image is currently named after the repository (erpnext_germany).
The workflow is currently triggered on:

• Pushes to the main branch
• Manual triggers via the GitHub Actions UI

But not if any of the custom apps change.

[revant]

• Do daily / nightly builds if it fits your case. We do it for frappe_docker for develop branches of frappe and erpnext https://github.com/frappe/frappe_docker/blob/main/.github/workflows/build_develop.yml. Helps us be prepared for upcoming changes and fix builds.
• Do set triggers for PRs that change build related files.