Executors docker edition

Created by: eseliger

This PR adds a docker version of executors.

Next up:

• Making it work for SSBC by allowing custom URLs for the SRC_ENDPOINT variable (this variable I want to get rid off soon, but otherwise I can't test it)
• Craft a k8s manifest to run in locally in the docker managed k8s to verify it can work
• Create a docker-compose extension that can run this successfully to verify it can also work for compose deployments

Test plan

Ran a main-dry-run and a executor-patch-notest build to validate they pass.