ci: enable BuildKit + let its GC manage the layer cache

DOCKER_BUILDKIT=1 (explicit) so the restore layer is cached across builds; drop the
explicit 'docker builder prune' (it was wiping that cache) and rely on BuildKit's own
GC + 86GB headroom. Cleanup keeps dangling-image removal only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -13,6 +13,7 @@ on:
 env:
   GIT_HOST: git.easysoft.ro
   REGISTRY_HOST: registry.easysoft.ro
+  DOCKER_BUILDKIT: "1"
   API_IMAGE: apps/myai-api
   CV_MATCHER_API_IMAGE: apps/myai-cv-matcher-api
   RAG_API_IMAGE: apps/myai-rag-api
@@ -109,4 +110,3 @@ jobs:
         if: always()
         run: |
           docker image prune -f   # dangling only (keep base images)
-          docker builder prune -f --filter until=336h   # keep ~2 weeks of layer cache