message: run_envoy_docker: don't pull latest when working with explicit image ID.
  docker pull takes a tag, not an image ID. Plus, the point of the IMAGE_ID override is to work with
  an image ID found in 'docker images', not to remote fetch.
diff: @@ -11,10 +11,15 @@ else
  USER_GROUP=$(id -g)
  fi
  +# The IMAGE_ID can be set to an arbitrary image ID (found with 'docker images').
  [[ -z "${IMAGE_ID}" ]] && IMAGE_ID="latest"
  [[ -z "${ENVOY_DOCKER_BUILD_DIR}" ]] && ENVOY_DOCKER_BUILD_DIR=/tmp/envoy-docker-build
  mkdir -p "${ENVOY_DOCKER_BUILD_DIR}"
  -docker pull lyft/envoy-build:"${IMAGE_ID}"
  +# When not operating on an existing image ID, fetch latest.
  +if [[ "${IMAGE_ID}" == "latest" ]]
  +then
  + docker pull lyft/envoy-build:latest
  +fi
  docker run -t -i -u "${USER}":"${USER_GROUP}" -v "${ENVOY_DOCKER_BUILD_DIR}":/build \
  -v "$PWD":/source lyft/envoy-build:"${IMAGE_ID}" /bin/bash -c "cd source && $*"
  