Airship specific implementation of KRM Function Specification

[kozhukalov]

Problem description (if applicable)
At the moment Airship uses two types of generic containers: krm (kyaml/runfn) and airship (airship specific docker wrapper). kyaml/runfn has some limitations that do not allow to use it everywhere. For example, sometimes we want a container to be run from a privileged user. Also kyaml/runfn does not provide some of the functionality we potentially would like to have (e.g. running timeout).

Proposed change
Instead of using two container runtimes, let's use just one. The suggestion is

1. Get rid of Airship specific docker client
2. Copy the code from kyaml/runfn and modify it to cover Airship specific needs

The goal is to have one container runtime that would implement KRM Function Specification https://github.com/kubernetes-sigs/kustomize/blob/master/cmd/config/docs/api-conventions/functions-spec.md#krm-functions-specification and have all necessary functionality needed for Airship.

The same approach is used in https://github.com/GoogleContainerTools/kpt. They copied kyaml/runfn and modified the code according to their needs.

[kozhukalov]

Please assign this to me

[kozhukalov]

https://review.opendev.org/q/topic:krm_function_spec