Bump to Kaniko 0.8

Author: dlorenc

ruby-pipeline/ruby-latest.yaml

@@ -44,7 +44,5 @@ steps:
            '--prebuilt-image=2.5.3=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.5.3:latest',
            '--prebuilt-image=2.6.0=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.0:latest',
            '--default-ruby-version=2.3.8']
-  - name: 'gcr.io/cloud-builders/docker:latest'
-    args: ['build', '--network=cloudbuild', '-t', '$_OUTPUT_IMAGE', '.']
-images:
-  - '$_OUTPUT_IMAGE'
+  - name: 'gcr.io/kaniko-project/executor:v0.8.0'
+    args: ['--destination=$_OUTPUT_IMAGE']