feat: Add multiplatform container image support (amd64, arm64, s390x)

[crivetimihai]

Add comprehensive multiplatform Docker build support with:

• New docker-multiplatform.yml workflow:

  • Parallel native builds for amd64 (ubuntu-latest) and arm64 (ubuntu-24.04-arm)
  • QEMU emulation for s390x on ubuntu-latest
  • Multiplatform manifest creation with buildx imagetools
  • Security scanning (Trivy, Grype, Syft SBOM) on amd64
  • Cosign keyless signing for all architectures

• Updated docker-release.yml:

  • Use buildx imagetools create for manifest handling
  • Preserves all architecture variants when tagging releases

• Updated ibm-cloud-code-engine.yml:

  • Explicit --platform linux/amd64 flag for consistent builds

• Updated Containerfile.lite for multiplatform compatibility:

  • Use ubi10-minimal as runtime base instead of scratch
  • Eliminates dnf --installroot which fails under QEMU emulation
  • Uses microdnf for runtime package installation
  • Maintains security scanning compatibility (RPM database preserved)

• Enhanced Makefile targets:

  • container-build-multi: Build multiplatform image locally
  • container-inspect-manifest: Inspect multiplatform manifest in registry

Closes #80