fix: pin Docker Build job to Linux runner

Docker Build was using bare `self-hosted` label, allowing it to run on
the macOS runner which has no Docker daemon. Pin to [self-hosted, Linux]
so it always runs on openmediavault.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jeffcrouse

.github/workflows/ci.yml

@@ -421,7 +421,7 @@ jobs:
 
   docker-build:
     name: Docker Build
-    runs-on: self-hosted
+    runs-on: [self-hosted, Linux]
     timeout-minutes: 30
     needs: [backend-lint, backend-test-contract, backend-test-core, backend-test-integration, frontend-lint, frontend-test]
     steps: