added kubectl to image

.drone.yml

@@ -1,17 +1,13 @@
 pipeline:
 
-  build:
-    image: alpine:edge
-    commands:
-      - sed -i 's@<VERSION>@${VERSION}@' Dockerfile
-
   docker:
     image: plugins/docker
     repo: bunix42/google-cloud-sdk
     build_args:
       - BASE_IMAGE=google/cloud-sdk:${VERSION}
+      - KUBECTL_VERSION=${KUBECTL_VERSION}
     tags:
-     - ${VERSION}
+     - ${VERSION}_kubectl-${KUBECTL_VERSION}
     secrets: [ docker_username, docker_password ]
     when:
       branch: master
@@ -19,3 +15,7 @@ pipeline:
 matrix:
   VERSION:
     - 193.0.0-alpine
+  KUBECTL_VERSION:
+    - v1.9.10
+    - v1.10.7
+    - v1.11.2

Dockerfile

@@ -3,3 +3,8 @@ FROM $BASE_IMAGE
 
 RUN apk add -U --no-cache openssl
 
+# download and install kubectl
+ARG KUBECTL_VERSION
+
+RUN curl -L https://storage.googleapis.com/kubernetes-release/release/$KUBECTL_VERSION/bin/linux/amd64/kubectl -o /usr/bin/kubectl \
+	&& chmod +x /usr/bin/kubectl