Add support for balenaEngine proxy configuration

[majorz]

balenaEngine/Docker allow specifying a proxy server: https://docs.docker.com/config/daemon/systemd/#httphttps-proxy. This can be used in conjunction with a project like https://github.com/rpardini/docker-registry-proxy to provide caching features for scenarios where there are multiple balena devices on a single network.

[jellyfish-bot]

[majorz] This has attached https://jel.ly.fish/b9c27c15-b966-4a21-92ef-9a93737a8b73