ci: specify DOCKER_CONFIG env variable for kaniko

kaniko does not appear to pick docker hub credentials placed at
`/root/.docker/` anymore. As a result the release stage was not able to
push images to the docker registry. In this PR we explicitly specify the
path to the docker `config.json` file in the `DOCKER_CONFIG` variable.

We've also updated the kaniko version in this PR.

(cherry picked from commit 65a0b02)

Jenkinsfile

@@ -180,8 +180,11 @@ spec:
     command:
     - 'cat'
   - name: 'kaniko'
-    image: 'gcr.io/kaniko-project/executor:debug-v0.8.0'
+    image: 'gcr.io/kaniko-project/executor:debug-v0.9.0'
     tty: true
+    env:
+    - name: 'DOCKER_CONFIG'
+      value: '/root/.docker'
     command:
     - '/busybox/cat'
     volumeMounts: