use custom type Tag for tag in config source

since json.Number doesn't allow to decode non-numeric string anymore.
golang/go#14702

The Tag.UnmarshalJSON() function is taken from registry-image-resource so
it is good that we finally make them consistent.

Add more tests to cover this change. Nothing is expected to be broken.

Signed-off-by: Rui Yang <ryang@pivotal.io>

cmd/check/main.go

@@ -65,7 +65,7 @@ func main() {
 		registryHost = registryMirrorUrl.Host
 	}
 
-	tag := request.Source.Tag.String()
+	tag := string(request.Source.Tag)
 	if tag == "" {
 		tag = "latest"
 	}

cmd/check/models.go

@@ -4,7 +4,7 @@ import "encoding/json"
 
 type Source struct {
 	Repository         string          `json:"repository"`
-	Tag                json.Number     `json:"tag"`
+	Tag                Tag             `json:"tag"`
 	Username           string          `json:"username"`
 	Password           string          `json:"password"`
 	InsecureRegistries []string        `json:"insecure_registries"`
@@ -38,3 +38,20 @@ type ClientCertKey struct {
 	Cert   string `json:"cert"`
 	Key    string `json:"key"`
 }
+
+// Tag refers to a tag for an image in the registry.
+type Tag string
+
+// UnmarshalJSON accepts numeric and string values.
+func (tag *Tag) UnmarshalJSON(b []byte) (err error) {
+	var s string
+	if err = json.Unmarshal(b, &s); err == nil {
+		*tag = Tag(s)
+	} else {
+		var n json.RawMessage
+		if err = json.Unmarshal(b, &n); err == nil {
+			*tag = Tag(n)
+		}
+	}
+	return err
+}

tests/check_test.go

@@ -54,7 +54,7 @@ var _ = Describe("Check", func() {
 		session := check(map[string]interface{}{
 			"source": map[string]interface{}{
 				"repository": repository,
-				"tag": tag,
+				"tag":        tag,
 			},
 		})
 
@@ -71,4 +71,26 @@ var _ = Describe("Check", func() {
 
 		Expect(session.Out).To(gbytes.Say(`{"digest":`))
 	})
+
+	It("prints out the digest for a existent image and quoted numeric tag", func() {
+		session := check(map[string]interface{}{
+			"source": map[string]interface{}{
+				"repository": "alpine",
+				"tag":        "3.7",
+			},
+		})
+
+		Expect(session.Out).To(gbytes.Say(`{"digest":`))
+	})
+
+	It("prints out the digest for a existent image and numeric tag", func() {
+		session := check(map[string]interface{}{
+			"source": map[string]interface{}{
+				"repository": "alpine",
+				"tag":        3.7,
+			},
+		})
+
+		Expect(session.Out).To(gbytes.Say(`{"digest":`))
+	})
 })