fix(docker): keep .env.local out of the build context

`**/*.env` never matched `frontend/.env.local`, so a local override pointing at
a remote backend was copied into the image. Exclude every `.env.*` and keep the
examples. Verified on a probe context: `.env` and `.env.local` are dropped,
`.env.example` survives.

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

.dockerignore

@@ -10,5 +10,7 @@ frontend/ssl
 
 **/*.log
 **/*.env
+**/.env.*
+!**/.env.example
 **/.DS_Store
 **/Thumbs.db