ci(ui): run the UI build check through the image's ui-builder stage

The build-ui check compiled the dashboard from a full checkout, so any
import reaching above ui/litellm-dashboard/ resolved there and only broke
inside the images, where the stage copies the dashboard tree alone.
Building the stage itself puts the check on the same file boundary the
shipped images use.

.github/workflows/test-litellm-ui-build.yml

@@ -19,9 +19,6 @@ jobs:
   build-ui:
     runs-on: ubuntu-latest
     timeout-minutes: 10
-    defaults:
-      run:
-        working-directory: ui/litellm-dashboard
 
     steps:
       - name: Checkout repository
@@ -35,18 +32,11 @@ jobs:
         with:
           category: ui
 
-      - name: Setup Node.js
+      # Built through the image stage rather than the checkout, because the
+      # stage copies ui/litellm-dashboard/ alone: an import reaching above the
+      # dashboard root resolves in a checkout and fails in every image we ship.
+      # Dockerfile, docker/Dockerfile.non_root and ui/Dockerfile share this
+      # stage verbatim, so building one covers all three.
+      - name: Build the dashboard as the shipped images build it
         if: steps.changes.outputs.decision != 'skip'
-        uses: actions/setup-node@a0853c24544627f65ddf259abe73b1d18a591444 # v5.0.0
-        with:
-          node-version-file: ui/litellm-dashboard/.nvmrc
-          cache: "npm"
-          cache-dependency-path: ui/litellm-dashboard/package-lock.json
-
-      - name: Install dependencies
-        if: steps.changes.outputs.decision != 'skip'
-        run: npm ci
-
-      - name: Build
-        if: steps.changes.outputs.decision != 'skip'
-        run: npm run build
+        run: docker build --target ui-builder -f Dockerfile .