Using the same (recent) Go version for all Kubernetes versions can
break for older versions when there are incompatible changes in Go. To
avoid that, we use exactly the minimum version of Go required for each
Kubernetes version. This is based on the assumption that this
combination was tested successfully.
When building the E2E suite from Kubernetes (the default) we do the
same, but still allow building it from elsewhere.
We allow the Go version to be empty when it doesn't matter.
This enables testing of other repos and of this repo itself inside
Prow. Currently supported is unit testing ("make test") and E2E
testing (either via a local test suite or the Kubernetes E2E test
suite applied to the hostpath driver example deployment).
The script passes shellcheck and uses Prow to verify that for future
PRs.
