No more go get.

It's been deprecated and removed. The alternative is go install but it's
not a direct replacement. Specifically, replace directives can't appear
in the go.mod file. Kustomize can be installed using go install, so
that's fine. RKE and reg can't, so use the GitHub CLI to download the
binaries from the latest release.

Dockerfile

@@ -228,9 +228,6 @@ RUN apt-get update && \
     export GOPATH=/usr/local/lib/go && \
     export PATH="$GOPATH/bin:$PATH" && \
     mkdir "$GOPATH" && \
-    go get sigs.k8s.io/kustomize/kustomize/v3 && \
-    go get github.com/rancher/rke && \
-    go get github.com/genuinetools/reg@v0.16.1 && \
     export GO111MODULE=on && \
     go install github.com/giantswarm/semver-bump@latest && \
     go install github.com/nishanths/license/v5@latest && \
@@ -239,10 +236,15 @@ RUN apt-get update && \
     go install github.com/lucagrulla/cw@latest && \
     go install github.com/kvz/json2hcl@latest && \
     go install github.com/hashicorp/envconsul@latest && \
+    go install sigs.k8s.io/kustomize/kustomize/v4@latest && \
     wget https://raw.githubusercontent.com/rabbitmq/rabbitmq-server/v3.8.16/deps/rabbitmq_management/bin/rabbitmqadmin -qO /usr/local/bin/rabbitmqadmin && \
     chmod +x /usr/local/bin/rabbitmqadmin && \
     mkdir /usr/local/share/bfg && \
     wget 'https://search.maven.org/remote_content?g=com.madgag&a=bfg&v=LATEST' -qO /usr/local/share/bfg/bfg.jar && \
+    gh release download --repo genuinetools/reg --pattern "reg-$(go env GOOS)-$(go env GOARCH)" && \
+    install -o root -g root -m 0755 /usr/local/bin/reg reg-* && \
+    gh release download --repo rancher/rke --pattern "rke_$(go envGOOS)-$(go env GOARCH)" && \
+    install -o root -g root -m 0755 /usr/local/bin/rke rke_* && \
     _PIPENV_COMPLETE=bash_source pipenv > /etc/bash_completion.d/pipenv && \
     rabbitmqadmin --bash-completion  > /etc/bash_completion.d/rabbitmqadmin && \
     poetry completions bash > /etc/bash_completion.d/poetry && \