cmd/k8s-operator: allow specifying base image in e2e tests (#20727)

This commit adds a new `--base-image` flag to the e2e test suite
so that tests can build the operator and operator accessories
with a different base docker image. We want this so that we can
try things out with red hat's UBI as part of getting the operator
up and running on openshift clusters.

We can then modify the e2e test runner to use a matrix for normal
alpine base and redhat's.

Updates: https://github.com/tailscale/corp/issues/45981
Updates: https://github.com/tailscale/corp/issues/44443

Signed-off-by: David Bond <davidsbond93@gmail.com>