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Update 0034-build-docker-with-kaniko.md

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Ferenc Hammerl
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# ADR 0034: Build container-action Dockerfiles with Kaniko
**Date**: 2022-09-29
**Date**: 2023-01-09
**Status**: In Progress
# Background
[Building Dockerfiles in k8s using Kaniko](https://github.com/actions/runner-container-hooks/issues/23) has been on the radar since the beginning of container hooks.
Currently, it is only possible in ARC using a [dind/docker-in-docker](https://github.com/actions-runner-controller/actions-runner-controller/blob/master/runner/actions-runner-dind.dockerfile) sidecar container. This container needs to be launched using `--privileged`, which presents a security vulnerability.
Currently, it is only possible in ARC using a [dind/docker-in-docker](https://github.com/actions-runner-controller/actions-runner-controller/blob/master/runner/actions-runner-dind.dockerfile) sidecar container.
This container needs to be launched using `--privileged`, which presents a security vulnerability.
As an alternative tool, a container running [Kaniko](https://github.com/GoogleContainerTools/kaniko) can be used to build these files instead.
Kaniko doesn't need to be `--privileged`.
Whether using dind/docker-in-docker sidecar or Kaniko, in this ADR I will refer to these containers as '**builder containers**'
# Guiding Principles
- **Security:** running a Kaniko builder container should be possible without the `--privileged` flag
- **Feature parity with Docker:** Any 'Dockerfile' that can be built with vanilla Docker should also be possible to build using a Kaniko build container
- **Function over form:** a limitation we have found with this approach is that in order for a container-action to be run as a k8s job
(which is how it works today with container-actions that specify a registry/image instead of a local Dockerfile that we need to build),
**the k8s job needs to pull the image to be executed from a registry**. In the initial iteration, we assume a user-configured registry that is either exposed as a k8s _service_ through a nodePort OR a public registry (dockerhub, ghcr).
- **Ease of Use:** The customer should be able to build and push Docker images with minimal configuration
## Interface
The user will set `containerMode:kubernetes` since this is a change to the behaviour of our k8s hooks
The user will
EITHER
- Provide a set of ENVs to a (cluster-local) docker registry into which the Kaniko builder container can push the image
- `ACTIONS_RUNNER_CONTAINER_HOOKS_REGISTRY_HOST` # Registry (service) name for kaniko where to push, e.g. 'docker-registry', kaniko pushes to this domain
- `ACTIONS_RUNNER_CONTAINER_HOOKS_REGISTRY_PORT` # Container (service) port of the registry, e.g. 5000, kaniko pushes through this port
- `ACTIONS_RUNNER_CONTAINER_HOOKS_REGISTRY_NODE_PORT` # NodePort - the K8S job will pull the image from here
- The hooks then build up a URI like `localhost:${registryNodePort()}/${generated-random-string-handle-image}` for the k8s job to pull from
- `localhost` is the current limitation enforcing the existence of a docker registry exposed through the `nodePort` of a k8s service
The user will set `containerMode:kubernetes`, because this is a change to the behaviour of our k8s hooks
OR
The user will set two ENVs:
- `ACTIONS_RUNNER_CONTAINER_HOOKS_K8S_REGISTRY_HOST`: e.g. `ghcr.io/OWNER` or `dockerhandle`.
- `ACTIONS_RUNNER_CONTAINER_HOOKS_K8S_REGISTRY_SECRET_NAME`: e.g. `docker-secret`: the name of the `k8s` secret resource that allows you to authenticate against the registry with the given handle above
- Define a remote docker registry (dockerhub, ghcr) and its secrets (WIP)
The workspace is used as the image name.
To execute a container-action, we run a k8s job by loading the image from the specified registry
(TBD) Our hooks will then remove the image from the registry
The image tag is a random generated string.
To execute a container-action, we then run a k8s job by loading the image from the specified registry
ENVs `ACTIONS_RUNNER_CONTAINER_HOOKS_K8S_REGISTRY_HOST_PUSH` and `ACTIONS_RUNNER_CONTAINER_HOOKS_K8S_REGISTRY_HOST_PULL` will be preferred if set.
Users may want to use different URLs for push and pull as they will be invoked by different machines on different networks.
- The Kaniko build container pushes the image after building is a pod that belongs to the runner pod.
- The kubelet then pulls the image.
The above two might not resolve all host names 100% the same (k8s services, nodeports etc) so it makes sense to allow different push and pull URLs.
## Limitations
- The user needs to provide a local Docker Registry or config for a remote registry (like ghcr or dockerhub)
- The user needs to provide a local Docker Registry within the k8s cluster or config for a remote registry (like ghcr or dockerhub)
- Potential incompatibilities / inconsistencies between Docker and Kaniko, none is known at this time
## Consequences