feat: Add OCI image descriptions for multi-arch

[iamimmanuelraj]

This pull request enhances the Docker images for Frappe Manager by adding standardized Open Container Initiative (OCI) metadata annotations and improving the build process for multi-architecture images. These changes make the images more informative and compliant with best practices, and improve the reliability of multi-arch manifest creation.

OCI metadata annotation improvements:

• Added OCI-compliant labels such as org.opencontainers.image.description, org.opencontainers.image.source, and org.opencontainers.image.title to Docker/frappe/Dockerfile (both build and final stages) and Docker/nginx/Dockerfile to provide better image documentation and provenance. [1] [2] [3]

Multi-architecture build enhancements:

• Introduced the IMAGE_DESCRIPTION variable and logic in Docker/build.sh to annotate the multi-arch manifest with a description, and added fallback mechanisms for manifest creation when buildx imagetools is unavailable. This improves the robustness of multi-arch image publishing and ensures OCI description annotations are included when possible. [1] [2]