feat(image): support tar.gz image (fanal#40)

utils/utils_test.go

@@ -0,0 +1,32 @@
+package utils
+
+import (
+	"os"
+	"testing"
+)
+
+func TestIsGzip(t *testing.T) {
+	var tests = []struct {
+		in   string
+		want bool
+	}{
+		{"testdata/test.txt.gz", true},
+		{"testdata/test.tar.gz", true},
+		{"testdata/test.txt", false},
+		{"testdata/test.txt.zst", false},
+		{"testdata/aqua.png", false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			f, err := os.Open(tt.in)
+			if err != nil {
+				t.Fatalf("unknown error: %s", err)
+			}
+
+			got := IsGzip(f)
+			if got != tt.want {
+				t.Errorf("got %t, want %t", got, tt.want)
+			}
+		})
+	}
+}