Is your feature request related to a problem? Please describe.
I would like to be able to test and integrate my custom version of agones from my private registry to my infrastructure.
This is currently possible to push the build images to a custom registry by using the REGISTRY env variable,
But there is not options to push the charts to a custom registry though, the Makefiles only gives you the option to push to a custom GCP_BUCKET_CHARTS

Describe the solution you'd like
It would be nice to have an option to also push the charts of the current dev version to a custom registry using helm push.
This would means to also update the version and registry within the helm charts.

Having an env variable CHART_REGISTRY that would use helm push ... and set the values from REGISTRY and the current version (dev
release) in the helm values,
If not set it would use the default gcp bucket charts

This would looks like this: REGISTRY=X CHART_REGISTRY=X make push-chart (with the version already set from /build/Makefile: VERSION ?= $(base_version)-dev-$(shell git rev-parse --short=7 HEAD))

Describe alternatives you've considered
I can create a custom Makefile in the /build/includes/X which would contains my custom make push-charts, but this might also be interesting for other people who wants to test their custom version of agones with their updated charts

Additional context
This would help me testing my custom agones version within my infra

Link to the Agones Feature Proposal (if any)
Will work on the AFP PR