Build and push the ubbagent image as part of cloudbuild.yml

[maelvls]

The "ubbagent" is a Go project that serves as a "building block" for companies to build their own billing agent. Because it is meant to be customized for each Marketplace solution, Google does not maintain an upstream image. Later on, we will need to customize our "ubb-agent" for things like anti-cheating.

See the marketplace docs: billing-integration.md

Right now, we don't care about anti-cheating; we just want the ubb-image to report to the Google Billing API the hourly use of the jetstack-secure-for-cert-manager solution when it is being run by a customer.

This issue is about forking and having an automated build of ubbissuer to some registry e.g.

quay.io/jetstack/jetstack-secure-google-ubbagent:0.1.0

or something like that.