fix: runner: ignore unknown file types and sockets (fixes canonical#176)

spread/runner.go

@@ -277,7 +277,7 @@ func (r *Runner) prepareContent() (err error) {
 		return fmt.Errorf("cannot remove temporary content file: %v", err)
 	}
 
-	args := []string{"c", "--exclude=.spread-reuse.*"}
+	args := []string{"c", "--exclude=.spread-reuse.*", "--warning=no-file-ignored"}
 	if r.project.Repack == "" {
 		args[0] = "cz"
 	}