Add Docker support (standalone Next.js output)

Multi-stage Dockerfile producing a minimal runtime image via
next.config.ts's output:"standalone". NEXT_PUBLIC_API_URL is taken as
a build arg since it gets baked into the client bundle at build time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,8 @@
+node_modules
+.next
+.git
+.env
+.env.local
+!.env.local.example
+npm-debug.log*
+uploads