fix(publish-fork): wrap digest file keys in an array

With a single published image PSObject.Properties.Name returns a scalar
string, so $keys[0] indexed its first character instead of the first
key, failing validation with a malformed-digest error.

Author: fdcastel

firebird-docker.build.ps1

@@ -524,7 +524,7 @@ task Publish-Manifests FilteredAssets, {
         } catch {
             throw "Digest file '$path' is not valid JSON: $($_.Exception.Message)"
         }
-        $keys = $parsed.PSObject.Properties.Name
+        $keys = @($parsed.PSObject.Properties.Name)
         if (-not $keys -or $keys.Count -eq 0) {
             throw "Digest file '$path' is empty — no digests to assemble."
         }