chore(rootfs/Dockerfile): update go toolchain to 1.16.3 (#278)

See https://golang.org/doc/go1.16
and https://github.com/golang/go/issues?q=milestone%3AGo1.16.3+label%3ACherryPickApproved

Author: mboersma

rootfs/Dockerfile

@@ -7,7 +7,7 @@ ENV ANSIBLE_VERSION=2.10.3 \
     AZCLI_VERSION=2.20.0 \
     DOCKER_VERSION=20.10.2 \
     ETCDCTL_VERSION=v3.1.8 \
-    GO_VERSION=1.15.11 \
+    GO_VERSION=1.16.3 \
     GOLANGCI_LINT_VERSION=v1.38.0 \
     HELM_VERSION=3.5.2 \
     KUBECTL_VERSION=v1.16.3 \