libgit2 does not support HTTP proxy for Git clone/fetch/push

[au2001]

The context of this issue is the same as fluxcd/source-controller#131.
My company is running a Kubernetes cluster behind a Zscaler corporate proxy.

As per stefanprodan's first comment on the issue above, I wrote a Kustomize patch which adds the environment variables containing the proxy information to all controllers' containers.

source-controller has a gitImplementation setting which allows to use either libgit2 or go-git (which is the default) but image-automation-controller does not follow that setting since #177 (released in v0.11.0).
The issue is that go-git is the only implementation between the two to work with the environment variables approach.
libgit2 does not honor them and instead has a dedicated configuration option. Unfortunately, source-controller and image-automation-controller do not expose it.

One solution would be to expose this option in source-controller when using the libgit2 implementation (fluxcd/source-controller#131) but that issue is unresolved since Sep 4, 2020.
Another solution would be to revert #177 and add back the option to use go-git for Git clone, fetch and push operations in image-automation-controller.

Related issue: fluxcd/flux2#1593