set proxy variables in host container environment

[bcressey]

What I'd like:
Host containers should receive the same HTTPS_PROXY and NO_PROXY variables we set for various services on the host.

This is particularly important for bootstrap containers (#1392) but would be generally useful. For orchestrated containers, we expect the correct values to be supplied by the orchestrator agent, but for these containers we're effectively taking on that role.

Any alternatives you've considered:
Pass the variables in through host container user data, or else bake them into the container image. Neither approach seems quite as elegant.