Local cache of public docker containers

[JorritSalverda]

The execution of the .estafette.yaml manifest completely depends on Docker container images. As the ci-builder runs docker-in-docker and starts a fresh Kubernetes job for every build the used Docker container images are never available locally and have to be fetched from Docker Hub.

To improve performance a local cache would help. Preferably with background refreshes of the dev, beta and stable track tags.

[JorritSalverda]

https://docs.docker.com/registry/recipes/mirror/