Add support for remote backends via VIRTUAL_IP

[f403]

This adds support for non-container upstreams (e.g. somewhere on LAN).
Some time ago I used a patch from #1100 to solve this issue, but current template has changed a lot.
Although running a dummy container for each custom upstream is a bit hacky, but it keeps same approach for all deployments, implements a feature requested many times, and does this with minimal changes in the template.

This PR may potentially close:

• Manually setting upstream #832
• No upstreams generated when container has it's own IP #1937
• Remote Server #1191
• Required VIRTUAL_UPSTREAM: support the web service not in docker.  #1474
• VIRTUAL_PORT is not filled in default.conf when container uses host networking #1270
• Bad Gateway --net=host #1537

Related PRs (some may be hard to merge now, if they are old):

• Non-Docker Host #1100
• support VIRTUAL_IP env variable #1219
• Support no Docker proxy by env VIRTUAL_UPSTREAM #1477
• Support containers running --net=host (#1537) #2222

[buchdag]

Hi.

Thanks for the PR.

Unfortunately while I do understand that's a wanted feature, my answer from mid 2021 still stands: this is out of the original stated scope of this project, and I don't see it implemented using zombie containers.