docs(docker): correct stale runtime-deps comment (no build toolchain anymore)

Dockerfile

@@ -38,7 +38,8 @@ FROM node:20-slim AS runtime
 
 RUN corepack enable && corepack prepare pnpm@10.34.3 --activate
 
-# Install build dependencies for better-sqlite3 native module
+# Runtime deps only: ffmpeg (media processing) + gosu (drop to non-root in the
+# entrypoint). No C toolchain — better-sqlite3 and sharp load prebuilt binaries.
 RUN apt-get update && \
     apt-get install -y --no-install-recommends ffmpeg gosu && \
     rm -rf /var/lib/apt/lists/*