chore: add `syntax` parser directive to Dockerfile (#72380)
## What?
Add [`syntax`](https://docs.docker.com/reference/dockerfile/#syntax)
parser directive to the first line of the
[Dockerfile](https://docs.docker.com/reference/dockerfile/).
## Why?
To declare the Dockerfile syntax version to use for the build.
If unspecified, [BuildKit](https://docs.docker.com/build/buildkit/) uses
a bundled version of the Dockerfile frontend.
## How?
Append `# syntax=docker.io/docker/dockerfile:1` to the top of the
[Dockerfile](https://docs.docker.com/reference/dockerfile/).
Co-authored-by: JJ Kasper <jj@jjsweb.site>
