chore(kubernetes): bump kubectl binary minor version (#5047)

- add note regarding targeted version
- per releases: https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.18.md

Co-authored-by: Eric Zimanyi <ezimanyi@google.com>

Dockerfile.slim

@@ -1,7 +1,8 @@
 FROM alpine:3.12
 LABEL maintainer="sig-platform@spinnaker.io"
 
-ENV KUBECTL_VERSION v1.17.13
+# KUBECTL_VERSION kept one minor version behind latest to maximise compatibility overlap
+ENV KUBECTL_VERSION v1.18.10
 
 RUN apk --no-cache add --update bash wget unzip openjdk11 'python2>2.7.9' && \
   wget -nv https://dl.google.com/dl/cloudsdk/release/google-cloud-sdk.zip && \