Change Kubelet container image publishing

* Build Kubelet container images internally and publish
to Quay and Dockerhub (new) as an alternative in case of
registry outage or breach
* Use our infra to provide single and multi-arch (default)
Kublet images for possible future use
* Docs: Show how to use alternative Kubelet images via
snippets and a systemd dropin (builds on #737)

Changes:

* Update docs with changes to Kubelet image building
* If you prefer to trust images built by Quay/Dockerhub,
automated image builds are still available with unique
tags (albeit with some limitations):
  * Quay automated builds are tagged `build-{short_sha}`
  (limit: only amd64)
  * Dockerhub automated builts are tagged `build-{tag}`
  and `build-master` (limit: only amd64, no shas)

Links:

* Kubelet: https://github.com/poseidon/kubelet
* Docs: https://typhoon.psdn.io/topics/security/#container-images
* Registries:
  * quay.io/poseidon/kubelet
  * docker.io/psdn/kubelet

docs/advanced/customization.md

@@ -174,3 +174,34 @@ module "nemo" {
 
 To customize low-level Kubernetes control plane bootstrapping, see the [poseidon/terraform-render-bootstrap](https://github.com/poseidon/terraform-render-bootstrap) Terraform module.
 
+## Kubelet
+
+Typhoon publishes Kubelet [container images](/topics/security.md#container-images) to Quay.io (default) and to Dockerhub (in case of a Quay [outage](https://github.com/poseidon/typhoon/issues/735) or breach). Quay automated builds also provide the option for fully verifiable tagged images (`build-{short_sha}`).
+
+To set an alternative Kubelet image, use a snippet to set a systemd dropin.
+
+```
+# host-image-override.yaml
+variant: fcos           <- remove for Flatcar Linux
+version: 1.0.0          <- remove for Flatcar Linux
+systemd:
+  units:
+    - name: kubelet.service
+      dropins:
+        - name: 10-image-override.conf
+          contents: |
+            [Service]
+            Environment=KUBELET_IMAGE=docker.io/psdn/kubelet:v1.18.3
+```
+
+```
+module "nemo" {
+  ...
+
+  worker_snippets = [
+    file("./snippets/host-image-override.yaml")
+  ]
+  ...
+}
+```
+