fix: add dockerfile syntax directive for heredocs

[Namanlashkari]

Problem

assets/Dockerfile uses RUN <<-EOF for the LuaRocks install. Docker daemons that still default to the dockerfile.v0 frontend parse the heredoc body as instructions and fail:

unknown instruction: BUILD_VERSION=$(

#730

Fix

Add # syntax=docker/dockerfile:1 as the first line so BuildKit pulls the modern frontend regardless of the daemon built-in.

Testing

Manual:

• Confirmed assets/Dockerfile starts with # syntax=docker/dockerfile:1
• Heredoc RUN <<-EOF block is unchanged

CI:

• Existing Pongo image-build CI should parse the Dockerfile on current BuildKit