Update build/auth documentation to document kubernetes.io/* secrets (#1098)

Since v0.4.0 release, knative/build supports additional secrets for
docker registries : `kubernetes.io/dockerconfigjson` and
`kubernetes/dockercfg`.

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

docs/build/auth.md

@@ -206,6 +206,62 @@ When this build executes, before steps execute, a `~/.docker/config.json` will
 be generated containing the credentials configured in the `Secret`, and these
 credentials are then used to authenticate with the Docker registry.
 
+## Kubernetes's Docker registry's secret
+
+Kubernetes defines two types of secrets for Docker registries :
+`kubernetes.io/dockercfg` and
+`kubernetes.io/dockerconfigjson`. Knative supports those secrets in
+addition to the one described above.
+
+1. Define a `Secret` from a Docker client configuration file, as
+   documented in [Pull an Image from a Private
+   Registry](https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/) 
+   
+   ```bash
+   kubectl create secret generic regcred \
+    --from-file=.dockerconfigjson=<path/to/.docker/config.json> \
+    --type=kubernetes.io/dockerconfigjson
+   ```
+
+1. Direct a `ServiceAccount` to use this `Secret`:
+
+	```yaml
+    apiVersion: v1
+    kind: ServiceAccount
+    metadata:
+      name: build-bot
+    secrets:
+      - name: regcred
+	```
+
+1.  Use that `ServiceAccount` in your `Build`:
+
+    ```yaml
+    apiVersion: build.knative.dev/v1alpha1
+    kind: Build
+    metadata:
+      name: build-with-basic-auth
+    spec:
+      serviceAccountName: build-bot
+      steps:
+      ...
+    ```
+
+1.  Execute the build:
+
+    ```shell
+    kubectl apply --filename secret.yaml --filename serviceaccount.yaml --filename build.yaml
+    ```
+
+When this build executes, before steps execute, a `~/.docker/config.json` will
+be generated containing the credentials configured in the `Secret`, and these
+credentials are then used to authenticate with the Docker registry.
+
+If both `kubernetes.io/*` and knative flavored basic authentication
+secret are provided, knative will merge the credentials from those
+two ; knative flavored credentials taking precendence over
+`kubernetes.io/dockerconfigjson` (or `kubernetes.io/dockercfg`) ones.
+
 ### Guiding credential selection
 
 A build might require many different types of authentication. For instance, a