Dockerfile + PKILL_PILOT support

• Dockerfile: This release add a Dockerfile so that you can package up the binary in a docker image. I've been using this as a initContainer in kubernetes, copying it to a scratch directory, and then having my job definitions automatically prefix the given command with this. Might be a niche, but worked well enough for the especially when coupled with PKILL_PILOT
• PKILL_PILOT environment variable: This will cause envoy-preflight to use pkill -INT pilot-agent as the kill command, rather than the normal method for envoy. This is only effective in a kubernetes pod if you set shareProcessNamespace: true on the pod spec.