Add secret with docker credentials to demo script of cluster-tasks-tests

The jib-maven test case passes when providing docker credentials also to the pipeline service account in the name space:

Create a secret with your docker credentials (where the *** are below):

$ cat dockerio.yaml
apiVersion: v1
kind: Secret
metadata:
  name: dockerio
  annotations:
    tekton.dev/docker-0: https://docker.io
type: kubernetes.io/basic-auth
stringData:
  username: ***
  password: ***

Make the following changes to the demo script:

$ git diff demo
diff --git a/demo b/demo
index 93c06ff..d04ebf3 100755
--- a/demo
+++ b/demo
@@ -147,6 +147,7 @@ demo.prepare() {
     oc delete pvc --all -n ${NAMESPACE} --ignore-not-found
     oc delete -f kn-apply/sa-role-binding.yaml -n ${NAMESPACE} --ignore-not-found
     oc delete -f ssh-key.yaml -n ${NAMESPACE} --ignore-not-found
+    oc delete -f dockerio.yaml -n ${NAMESPACE} --ignore-not-found
     oc delete -f git-cli/sa.yaml -n ${NAMESPACE} --ignore-not-found
     oc delete -f git-cli/rolebinding.yaml -n ${NAMESPACE} --ignore-not-found
     oc delete -f maven/configmap.yaml -n ${NAMESPACE} --ignore-not-found
@@ -158,6 +159,8 @@ demo.prepare() {
   oc apply -f imagestream.yaml -n ${NAMESPACE}
   oc apply -f ssh-key.yaml -n ${NAMESPACE}
   oc secret link serviceaccount/pipeline secrets/ssh-key -n ${NAMESPACE}
+  oc apply -f dockerio.yaml -n ${NAMESPACE}
+  oc secret link serviceaccount/pipeline secrets/dockerio -n ${NAMESPACE}
   set -e
 }