Update from coreos/flannel-cni to poseidon/flannel-cni

* Update CNI plugins from v0.6.0 to v0.8.6 to fix several CVEs
* Update the base image to alpine:3.12
* Use `flannel-cni` as an init container and remove sleep
* poseidon/terraform-render-bootstrap#205
* https://github.com/poseidon/flannel-cni
* https://quay.io/repository/poseidon/flannel-cni

Background

* Switch from github.com/coreos/flannel-cni v0.3.0 which was last
published by me in 2017 and is no longer accessible to me to maintain
or patch
* Port to the poseidon/flannel-cni rewrite, which releases v0.4.0
to continue the prior release numbering

bootstrap.tf

@@ -1,6 +1,6 @@
 # Kubernetes assets (kubeconfig, manifests)
 module "bootstrap" {
-  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=45053a62cb751e2f8b371c20b7714e1604d6e57d"
+  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=3675b3a539efd8d341277f0c03322883f97fd992"
 
   cluster_name = var.cluster_name
   api_servers  = [format("%s.%s", var.cluster_name, var.dns_zone)]