.github/ci: Add nix-docker test

We still have room to spare in vm_tests, as it's quicker than `nix flake check`

.github/workflows/ci.yml

@@ -194,7 +194,13 @@ jobs:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
       - uses: DeterminateSystems/magic-nix-cache-action@main
-      - run: nix build -L .#hydraJobs.tests.githubFlakes .#hydraJobs.tests.tarballFlakes .#hydraJobs.tests.functional_user
+      - run: |
+          nix build -L \
+            .#hydraJobs.tests.functional_user \
+            .#hydraJobs.tests.githubFlakes \
+            .#hydraJobs.tests.nix-docker \
+            .#hydraJobs.tests.tarballFlakes \
+            ;
 
   flake_regressions:
     needs: vm_tests