fix: include package.json in Docker plugin directory

Headlamp's plugin discovery requires both main.js and package.json in
the plugin directory. The Dockerfile only copied dist/ (main.js),
causing the plugin to not be discovered at runtime.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -7,3 +7,4 @@ RUN npx @kinvolk/headlamp-plugin build
 
 FROM alpine:3.20
 COPY --from=build /app/dist/ /plugins/polaris-headlamp-plugin/
+COPY --from=build /app/package.json /plugins/polaris-headlamp-plugin/