use 2 stage Docker build to shrink final invoker-agent image size

docker/invoker-agent/Dockerfile

@@ -1,33 +1,20 @@
-from ubuntu:latest
+# build-stage
+FROM golang:alpine AS build-env
 
-ENV DOCKER_VERSION 1.12.0
-ENV KUBERNETES_VERSION 1.6.4
-
-RUN apt-get -y update && apt-get -y install \
-  wget \
-  git \
-  golang-go
-
-# Install docker client (for intractive debugging; not actually needed by agent)
-RUN wget --no-verbose https://get.docker.com/builds/Linux/x86_64/docker-${DOCKER_VERSION}.tgz && \
-tar --strip-components 1 -xvzf docker-${DOCKER_VERSION}.tgz -C /usr/bin docker/docker && \
-tar --strip-components 1 -xvzf docker-${DOCKER_VERSION}.tgz -C /usr/bin docker/docker-runc && \
-rm -f docker-${DOCKER_VERSION}.tgz && \
-chmod +x /usr/bin/docker && \
-chmod +x /usr/bin/docker-runc
-
-# Install kubernetes client (for intractive debugging; not actually needed by agent)
-RUN wget --no-verbose https://storage.googleapis.com/kubernetes-release/release/v${KUBERNETES_VERSION}/bin/linux/amd64/kubectl && \
-chmod +x kubectl && \
-mv kubectl /usr/bin/kubectl
+RUN apk add --no-cache git openssh
 
 RUN mkdir -p /openwhisk/src/invoker-agent
+COPY main.go /openwhisk/src/invoker-agent
 
 ENV GOPATH=/openwhisk
+RUN go get github.com/gorilla/mux
+RUN go install invoker-agent
 
-COPY main.go /openwhisk/src/invoker-agent
 
-RUN go get --insecure github.com/gorilla/mux
-RUN go install invoker-agent
+# Final stage
+FROM alpine
+
+RUN mkdir -p /openwhisk/src/invoker-agent
+COPY --from=build-env /openwhisk/bin/invoker-agent /openwhisk/bin/invoker-agent
 
 CMD ["/openwhisk/bin/invoker-agent"]