add unit test.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

docker_test.go

@@ -23,3 +23,45 @@ func Test_stripHeadPrefix(t *testing.T) {
 		}
 	}
 }
+
+func TestUseDefaultTag(t *testing.T) {
+	type args struct {
+		ref           string
+		defaultBranch string
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			name: "latest tag for default branch",
+			args: args{
+				ref:           "refs/heads/master",
+				defaultBranch: "master",
+			},
+			want: true,
+		},
+		{
+			name: "build from tags",
+			args: args{
+				ref:           "refs/tags/v1.0.0",
+				defaultBranch: "master",
+			},
+			want: true,
+		},
+		{
+			name: "skip build for not default branch",
+			args: args{
+				ref:           "refs/heads/develop",
+				defaultBranch: "master",
+			},
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		if got := UseDefaultTag(tt.args.ref, tt.args.defaultBranch); got != tt.want {
+			t.Errorf("%q. UseDefaultTag() = %v, want %v", tt.name, got, tt.want)
+		}
+	}
+}