ci: revert DOCKER_BUILDKIT (runner job env has no buildx plugin -> build failed)

Keep the cache win (no base-image nuke); back to the legacy builder which the job
context supports. BuildKit needs buildx installed in the runner before it can be used.

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

.gitea/workflows/build.yml

@@ -13,7 +13,6 @@ 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