Add kubeval installation

Author: Minku Lee

Dockerfile

@@ -1,8 +1,14 @@
 FROM gcr.io/cloud-builders/kubectl
 
+# Install Kubeval
+ADD https://github.com/garethr/kubeval/releases/download/0.6.0/kubeval-linux-amd64.tar.gz /tmp/kubeval.tar.gz
+RUN tar xf /tmp/kubeval.tar.gz -C /usr/local/bin && rm /tmp/kubeval.tar.gz;
+
+# Install Ruby 2.4
 RUN add-apt-repository ppa:brightbox/ruby-ng -y
 RUN apt-get update && apt-get install ruby2.4 ruby2.4-dev -y && apt-get clean
 
+# Install Bundler
 RUN gem update --system && gem install bundler
 
 ENTRYPOINT ["/bin/bash"]