Fix scratch setup image builder switch forwarding

devops/docker/build-all.ps1

@@ -55,6 +55,11 @@ if ([string]::IsNullOrWhiteSpace($RuntimeImage)) {
     $RuntimeImage = if ([string]::IsNullOrWhiteSpace($env:RUNTIME_IMAGE)) { 'mcr.microsoft.com/dotnet/aspnet:10.0-noble' } else { $env:RUNTIME_IMAGE }
 }
 
+if ($Registry.StartsWith('-')) {
+    Write-Error "Registry value '$Registry' is invalid. Invoke build-all.ps1 with named parameters so switches are not passed positionally."
+    exit 1
+}
+
 $Root = git rev-parse --show-toplevel 2>$null
 if (-not $Root) {
     Write-Error 'Not inside a git repository.'