Both builds in the same job shared the same docker buildx instance,
causing the backend image to be incorrectly tagged with the frontend image.
Separate jobs get isolated buildx instances and separate build caches.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds a second build+push step for the backend image (valknar/sexy-backend)
using Dockerfile.backend. Both images share the same tag strategy and
separate build caches. Summary step updated to show both images.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>