dockerEnabled for Stricter Clusters like OpenShift

[onelapahead]

Opening a draft while I test this out.

Motivation

I would like it so my users can make Runners without Docker access since on OpenShift that requires a SecurityContextConstraints. Other K8s clusters might require a PodSecurityPolicy instead FWIW.

Desired Experience

If users attempt to make a Runner with dockerEnabled: true and they don't have an SCC for the ServiceAccount they specified using serviceAccountName it will fail to create a pod and this is reflected in the Runner's status (this already occurs today without the dockerEnabled option).

If dockerEnabled: false is set, the user will then be able to have Runners create Pods without having to make an SCC/PSP because the Docker sidecar and volumes are omitted.

[Hi-Fi]

Isn't this same than new setting dockerdWithinRunnerContainer?

[Warashi]

when set dockerdWithinRunnerContainer true, runner container runs with privileged.
this PR maybe useful for users who doesn't need docker and want not to run privileged container.

[Warashi]

@hfuss
Do you have motivation to continue this PR?
If you don't, I will send PR doing same as this.

[onelapahead]

@Warashi I've had to put this down so please go ahead! Thank you for asking.

Will happily review and QA when it's ready.

[mumoshu]

@hfuss #191 has been released as a part of v0.12.0. Would you mind giving it a shot?

Thanks for the PR @Warashi!