fix(project): drop external dockerfile frontend directive

The `# syntax=docker/dockerfile:1` line made BuildKit fetch its frontend
image from Docker Hub on every build, even when all base images and layers
were already cached. On a host that briefly can't resolve registry-1.docker.io
this is the first and only mandatory network round-trip, so the build fails at
"resolve image config for docker-image://docker.io/docker/dockerfile:1" before
any stage runs.

This Dockerfile uses no frontend-specific syntax (no heredocs, no RUN --mount,
no COPY --link) — only multi-stage, COPY --from/--chown, RUN, ENV, etc., all
handled by the engine's built-in frontend. Dropping the directive removes the
Docker Hub dependency and lets a fully cached build complete offline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile

@@ -1,5 +1,3 @@
-# syntax=docker/dockerfile:1
-
 # =============================================================================
 # Tanabata File Manager — single-image build
 #