fix: multi-arch build

[alambike]

Closes #213

This pull request updates the image build workflow to better handle multi-platform builds by distinguishing between platforms that are actively built and those that are only included as variants in the published manifest. The main change is the introduction of a platforms_to_build parameter, allowing the Dagger app to search for multi-arch manifests in the registry, with the same image tag, and include those already uploaded images to the manifest.

Multi-platform build and publish improvements:

• Updated compile_image_and_publish in build_images.py to accept both platforms_to_build (platforms to actually build) and platforms (all platforms to include in the manifest). Platforms not built are logged and added as variants using the existing image, with error handling if a variant cannot be created.
• Adjusted the call to compile_image_and_publish in compile_images_for_all_flavors to pass both platforms_to_build and platforms as arguments, ensuring the new logic is used throughout the workflow.

Testing updates:

• Updated the test in test_build_images_functionality.py to use the new platforms_to_build parameter and ensure the test covers the new function signature and logic. [1] [2]

Minor cleanup:

• Removed an unnecessary blank line from the login method for code tidiness.