[Ingest Manager] Fix docker build for agent (#20268)

dev-tools/mage/dockerbuilder.go

@@ -103,7 +103,7 @@ func (b *dockerBuilder) copyFiles() error {
 		target := filepath.Join(b.beatDir, f.Target)
 		if err := Copy(f.Source, target); err != nil {
 			if f.SkipOnMissing && errors.Is(err, os.ErrNotExist) {
-				return nil
+				continue
 			}
 			return errors.Wrapf(err, "failed to copy from %s to %s", f.Source, target)
 		}