update Go version and parameterize Dockerfile

.drone.yml

@@ -188,6 +188,7 @@ steps:
   environment:
     GCLOUD_AUTH:
       from_secret: gcloud_auth
+    GOLANG: golang:1.16.2-alpine3.12
   commands:
   - dapper ci
   - echo "${DRONE_TAG}-arm" | sed -e 's/+/-/g' >.tags

Dockerfile.dapper

-FROM golang:1.15.8-alpine3.12
+ARG GOLANG=golang:1.15.8-alpine3.12
+FROM ${GOLANG}
 
 ARG http_proxy=$http_proxy
 ARG https_proxy=$https_proxy
@@ -22,10 +23,8 @@ RUN if [ "$(go env GOARCH)" = "arm64" ]; then
     tar -zxvf trivy_0.11.0_Linux-64bit.tar.gz                                                            && \
     mv trivy /usr/local/bin;                                                                                \
     fi
-RUN mkdir -p /go/src/golang.org/x && \
-    cd /go/src/golang.org/x && git clone https://github.com/golang/tools && cd tools && \
-    git checkout -b current aa82965741a9fecd12b026fbb3d3c6ed3231b8f8 && \
-    go install golang.org/x/tools/cmd/goimports && cd
+# this works for both go 1.15 and 1.16
+RUN GO111MODULE=on go get golang.org/x/tools/cmd/goimports@aa82965741a9fecd12b026fbb3d3c6ed3231b8f8
 RUN rm -rf /go/src /go/pkg
 
 RUN if [ "$(go env GOARCH)" = "amd64" ]; then \
@@ -37,7 +36,7 @@ ENV SELINUX $SELINUX
 
 ENV GO111MODULE off
 ENV DAPPER_RUN_ARGS --privileged -v k3s-cache:/go/src/github.com/rancher/k3s/.cache -v trivy-cache:/root/.cache/trivy
-ENV DAPPER_ENV REPO TAG DRONE_TAG IMAGE_NAME SKIP_VALIDATE GCLOUD_AUTH GITHUB_TOKEN
+ENV DAPPER_ENV REPO TAG DRONE_TAG IMAGE_NAME SKIP_VALIDATE GCLOUD_AUTH GITHUB_TOKEN GOLANG
 ENV DAPPER_SOURCE /go/src/github.com/rancher/k3s/
 ENV DAPPER_OUTPUT ./bin ./dist ./build/out
 ENV DAPPER_DOCKER_SOCKET true

Dockerfile.manifest

-FROM golang:1.15.8-alpine3.12
+ARG GOLANG=golang:1.15.8-alpine3.12
+FROM ${GOLANG}
 
 COPY --from=plugins/manifest:1.2.3 /bin/* /bin/
 

Dockerfile.test.dapper

-FROM golang:1.15.8-alpine3.12
+ARG GOLANG=golang:1.15.8-alpine3.12
+FROM ${GOLANG}
 
 RUN apk -U --no-cache add bash git gcc musl-dev docker curl jq coreutils python2 openssl