Support git checkout over http proxy

[davidkarlsen]

Describe the feature
See https://gitolite.com/git-over-proxy.html
If socat could be included in the image then checkout against github would be supported for environments locked down behind a corporate proxy. This would also mean exposing an option to configure /root/.ssh/config via the helm-chart.

What would the new user story look like?
How would the new interaction with Flux look like? E.g.
0. install flux

1. configure ssh-config to use socat

Feel free to add a diagram if that helps explaining things.

Expected behavior
Checkouts would go via http-proxy

[davidkarlsen]

Another option would be to checkout over https as normally but support username + token as described in https://help.github.com/en/articles/creating-a-personal-access-token-for-the-command-line

[hiddeco]

Besides to adding socat, the GIT_PROXY_COMMAND environment variable also needs to be whitelisted.

https://github.com/weaveworks/flux/blob/master/git/operations.go#L28