fix: allow .env.example in Docker build

The Docker build was failing because .dockerignore had ".env.*" which
excluded .env.example that the Dockerfile needs to copy.

Added exception rule "\!.env.example" to explicitly allow this file
while still excluding other .env.* files.

This fixes the GitHub Actions Docker build failure.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.dockerignore

@@ -5,7 +5,9 @@ npm-debug.log
 .gitignore
 .env
 .env.local
+# Exclude all .env.* files except .env.example
 .env.*
+!.env.example
 # Keep nodes.db but exclude other database files
 data/*.db
 data/*.db-*