Remove unnecessary Dockerfile syntax directive

[lbussell]

BuildKit is the default builder since Docker/Moby 23.0, and imagebuilder runs using moby-engine 25.0.3 on Azure Linux 3.0. The syntax directive is no longer needed. Removing it avoids pulling a remote image under network isolation. This is intended to fix build errors in build#2948160:

#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 254B done
#1 DONE 0.0s

#2 resolve image config for docker.io/docker/dockerfile:1
#2 ERROR: failed to do request: Head "[https://registry-1.docker.io/v2/docker/dockerfile/manifests/1"](https://registry-1.docker.io/v2/docker/dockerfile/manifests/1%22): dial tcp 192.0.2.32:443: i/o timeout
------
 > resolve image config for docker.io/docker/dockerfile:1:
------
Dockerfile:1
--------------------
   1 | >>> # syntax=docker/dockerfile:1
   2 |     ARG REPO=mcr.microsoft.com/dotnet/sdk
   3 |     FROM $REPO:10.0.201-azurelinux3.0-amd64
--------------------

[lbussell]

/backport to release/2026-04B

[github-actions]

Started backporting to release/2026-04B (link to workflow run)