feat: run nightly tests in GKE

[alexghr]

This PR moves the nightly tests from being run locally, in Kind, to running against a remote GKE cluster.

Right now each test deploys a whole network. This is slow and could cause contention if a bunch of pods fight over the same physical nodes (and autoscaling takes time).

Next item on the list: share a single deployment between all tests.

Docker release change: nightly runs will push the image early (tagged with $GIT_SHA) to docker hub in order for GKE to have access to it. After all tests pass the image is tagged with semver.