ci(containers): bake postgres extra into published backend image (#4015)

The tag-publish workflow (container.yaml) built backend/Dockerfile with no
build-args, so UV_EXTRAS was empty and the published *-backend image shipped
without the Postgres driver (only --extra redis). Multi-replica deployments
(K8s/Helm) that need shared Postgres persistence instead of file-based SQLite
could not use the release image without rebuilding it.

Pass UV_EXTRAS=postgres so the release image includes
deerflow-harness[postgres]. Additive only: single-replica sqlite/redis setups
keep working; the Postgres driver is added, mirroring how redis is already
always baked in.

.github/workflows/container.yaml

@@ -51,6 +51,12 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          # Bake the `postgres` extra into the published image so multi-replica
+          # deployments (K8s/Helm) can use shared Postgres persistence instead of
+          # file-based SQLite. sqlite/redis-only single-replica setups still work;
+          # this only adds the Postgres driver. See backend/Dockerfile `UV_EXTRAS`.
+          build-args: |
+            UV_EXTRAS=postgres
 
       - name: Generate artifact attestation
         uses: actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be #v2.4.0