Add support for private repos (add openssh-client)

Reduce docker image size

Add Docker tags in README.md

Add support for defining runner labels. Update README.md.

Add git 2.26.2

Support organization-level runners

Add package build-essential (fixes tcardonne#12)

Introduce GITHUB_ACCESS_TOKEN (Personnal Access Token) env var

Use slim version of debian buster

Use runner version 2.163.1 from GitHub