Drop the Dockerfile syntax directive

It made every build pull docker/dockerfile from Docker Hub, which fails
on hosts with rejected Hub credentials. The Dockerfile uses nothing the
built-in frontend lacks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1
 
 FROM golang:1-alpine AS build
 WORKDIR /src