Go Add Border
func addBorder(picture []string) []string {
	border := strings.Repeat("*", len(picture[0]))

	result := make([]string, 0, len(picture)+2)
	result = append(result, border)
	for _, line := range picture {
		result = append(result, "*"+line+"*")
	}
	result = append(result, border)

	return result
}

This builds a new grid with a * border around every side. It adds a full top and bottom row, then wraps each existing row from left and right.