Add missing .dockerignore

Without it, COPY . . in the Dockerfile overwrote the container's npm-ci
node_modules with the host's — a native-binary mismatch that surfaced
as a missing libnode.so.115 during the build-time page-data pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QcXH24ky6zjk2UyK5oZUPH

.dockerignore

@@ -0,0 +1,8 @@
+node_modules
+.next
+.git
+data
+*.env
+!.env.example
+coverage
+*.tsbuildinfo