benchmarks/README.md mentions oschaaf/benchmark-dev:latest as a location to source docker images from, which isn't right. We push images to https://hub.docker.com/r/envoyproxy/nighthawk-dev from CI, and we should try to consolidate a proper location for this, and push benchmark-dev images to it from CI.

Context:
[1] #609 (comment)