message: Allow specifying the entire image name in docker_build.sh
diff: #!/bin/bash
  [[ -z "${LINUX_DISTRO}" ]] && LINUX_DISTRO="ubuntu"
  +[[ -z "${IMAGE_NAME}" ]] && IMAGE_NAME="lyft/envoy-build-${LINUX_DISTRO}"
  # We need //bazel/... and WORKSPACE for the build, but it's not in ci/build_container. Using Docker
  # relative path workaround from https://github.com/docker/docker/issues/2745#issuecomment-253230025
  # to get this to work.
  tar cf - . -C ../../ bazel WORKSPACE \
  - | docker build -f Dockerfile-${LINUX_DISTRO} -t lyft/envoy-build-${LINUX_DISTRO}:$CIRCLE_SHA1 -
  + | docker build -f Dockerfile-${LINUX_DISTRO} -t ${IMAGE_NAME}:$CIRCLE_SHA1 -
  