build: opt to set progress warnings in response

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

util/confutil/metadata.go

@@ -39,3 +39,12 @@ func ParseMetadataProvenance(inp string) MetadataProvenanceMode {
 	}
 	return MetadataProvenanceModeMin
 }
+
+// MetadataWarningsEnabled returns whether metadata warnings are enabled from
+// BUILDX_METADATA_WARNINGS environment variable (default false)
+func MetadataWarningsEnabled() bool {
+	if ok, err := strconv.ParseBool(os.Getenv("BUILDX_METADATA_WARNINGS")); err == nil {
+		return ok
+	}
+	return false
+}