zuul: Use ./ for PUBLIC_URL when building in CI

Change-Id: I21fd853ad2831cc6fe9b1e873e7c412f7114e3f4

zuul-playbooks/build-dashboard.yaml

@@ -5,3 +5,10 @@
     - set-zuul-log-path-fact
     - role: npm
       npm_command: build
+  environment:
+    PUBLIC_URL: './'
+    # NOTE: using "./" is not enough to support html5 links, even with
+    # rewrite rules for unknown files, accessing 'job/devstack' will make
+    # the dashboard load static files from 'job/static/...'
+    # This works for the preview dashboard that can only be loaded from the
+    # npm/html directory anyway.