Docker layer keyserver handling for ppas is not proxy-aware, relies on apt-key and hkp

[Dmitrii Shcherbakov]

Currently this layer first tries to use a key id to retrieve keys and uses hkp://keyserver.ubuntu.com:80 as a default value for apt-key-server which is insecure.

https://github.com/juju-solutions/layer-docker/blob/4272d1a/reactive/docker.py#L356-L362
https://github.com/juju-solutions/layer-docker/blob/4272d1a/reactive/docker.py#L553-L565

    add_apt_key('9DC858229FC7DD38854AE2D88D81803C0EBFCD88')

Then it also tries to add a key by using a URL (add_apt_key_url via https://nvidia.github.io/nvidia-container-runtime/gpgkey), however, this is done by simply invoking curl without passing proxy settings explicitly.

The same applied to cuda driver handling.

See this pull request https://github.com/juju/charm-helpers/pull/248 for more information on juju-prefixed proxy settings, keyserver handling, apt-key deprecation.

The expectation is that configuring juju-http-proxy and juju-https-proxy model-configs will result in a properly deployed charm that will use this layer.