Provide GetIstio CLI in a container

[ZackButcher]

We've had some requests from folks for both GetIstio and GetEnvoy CLIs in containers so they can be more easily consumed by things like CI/CD systems. We should offer a GetIstio download as a container in addition to the current binary download for these use cases.

I believe if we stick the getistio binary into the container as the entrypoint, we'd be able to do stuff like: docker run somehub/getistio fetch --version 1.9 --flavor istio