examples: use dockerconfigjson type for kpack

it turns out that the use of `basic-auth` doesn't play nice with
imagePullSecrets - the fetching of registry credentials from
`basic-auth`-typed secrets is somethign that just tekton/kpack
controllers themselves deal with, but not kubelet

Signed-off-by: Ciro S. Costa <ciroscosta@vmware.com>

examples/source-to-knative-service/00-cluster/secret.yaml

@@ -13,14 +13,14 @@
 # limitations under the License.
 
 #@ load("@ytt:data", "data")
+#@ load("@ytt:json", "json")
+
 ---
 apiVersion: v1
 kind: Secret
 metadata:
   name: registry-credentials
-  annotations:
-    kpack.io/docker: #@ data.values.registry.server
-type: kubernetes.io/basic-auth
+type: kubernetes.io/dockerconfigjson
 stringData:
-  username: #@ data.values.registry.username
-  password: #@ data.values.registry.password
+  #@ registry_creds = {"username": data.values.registry.username, "password": data.values.registry.password}
+  .dockerconfigjson: #@ json.encode({"auths": {data.values.registry.server: registry_creds}})