# ADR 0034: Build container-action Dockerfiles with Kaniko

**Date**: 2022-09-29

**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.

As an alternative tool, a container running [Kaniko](https://github.com/GoogleContainerTools/kaniko) can be used to build these files instead.
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).

## 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
OR
- Define a remote docker registry (dockerhub, ghcr) and its secrets (WIP)

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


## Limitations
- The user needs to provide a local Docker Registry (TBD on supporting ghcr or dockerhub)
- Potential incompatibilities / inconsistencies between Docker and Kaniko, none is known at this time

## Consequences
- Users can now run AND build container-actions with a local Dockerfile in their k8s cluster