build: drop external Dockerfile frontend to avoid mirror pulls

backend/Dockerfile

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1
 # Build context: the repository backend/ directory.
 #   docker build -f backend/Dockerfile -t REGISTRY/PROJECT/zhinian-go-api:TAG backend/
 #

backend/Dockerfile.runtime

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1
 # Runtime-only image for build environments whose registry mirror cannot serve
 # the golang builder image. Build the static binary first (any machine with
 # Go 1.21, no Docker required), then assemble this image from alpine only: