Use bash

Dockerfile

@@ -1,8 +1,8 @@
 FROM alpine
-RUN apk -Uuv add curl ca-certificates
+RUN apk -Uuv add curl ca-certificates bash
 RUN curl -LO https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/linux/amd64/kubectl
 RUN chmod +x ./kubectl
 RUN mv ./kubectl /usr/local/bin/kubectl
 ADD update.sh /bin/
 RUN chmod +x /bin/update.sh
-CMD /bin/update.sh
+CMD bash -c /bin/update.sh

update.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 if [ -z ${PLUGIN_NAMESPACE} ]; then
   PLUGIN_NAMESPACE="default"