Add .dockerignore to exclude data dir from image builds

Without this, local spelling.db was being baked into the image,
causing the bind mount to appear empty or stale on first start.

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

.dockerignore

@@ -0,0 +1,7 @@
+node_modules
+data
+tts-cache
+.env
+*.db
+*.db-shm
+*.db-wal