executors: Improve release process
Created by: eseliger
This PR adds image families and well-known names to executor images. We now distinguish two modes:
- nightly
- tagged release Regular users of the terraform modules should not need to care about this at all, we will bake this into the modules. Generally, tagged releases make sure that we actually use the executor binary at the exact git tag. Nightly will be used in k8s for dogfooding. We can use a data source on the other end to fetch the images then, and don't need to copy around AMIs anymore all the time.
More to come here, but this'll be the foundation for https://github.com/sourcegraph/terraform-aws-executors/pull/42 and https://github.com/sourcegraph/terraform-google-executors/pull/56.
Test plan
Verified on a dry run branch and tested against k8s.