sketch: print Dockerfile contents

It's a bit of inside baseball, but building the Docker image
is slow enough that it'd be nice to stare at what it's doing,
even in the simplest form.
diff --git a/dockerimg/dockerimg.go b/dockerimg/dockerimg.go
index ae18bba..1aeb7e8 100644
--- a/dockerimg/dockerimg.go
+++ b/dockerimg/dockerimg.go
@@ -636,6 +636,11 @@
 	cmd.Stdout = stdout
 	cmd.Stderr = stderr
 	fmt.Printf("building docker image %s...\n", imgName)
+	dockerfileContent, err := os.ReadFile(dockerfilePath)
+	if err != nil {
+		return "", fmt.Errorf("failed to read Dockerfile: %w", err)
+	}
+	fmt.Printf("Dockerfile:\n%s", string(dockerfileContent))
 
 	err = run(ctx, "docker build", cmd)
 	if err != nil {