message: docs: update lyft/envoy-build readme
diff: After you have made changes to `build_container.sh` and merge them to master:
  1. Checkout master and pull latest changes.
  -2. Get the SHA of the master commit of your changes to `build_container.sh`.
  -3. From `~/envoy/ci/build_container` run `update_build_container.sh`. **Make sure to have
  +2. From `~/envoy/ci/build_container` run `update_build_container.sh`. **Make sure to have
  DOCKER_USERNAME and DOCKER_PASSWORD environment variables set**. This script will build
  - the envoy-build container with the current state of `build_container.sh`, tag the image
  - with the SHA provided, and push it to Dockerhub.
  -4. After you have done that, update `ci/ci_steps.sh` to pull the new tagged version of `lyft/envoy-build`.
  + the envoy-build container with the current state of `build_container.sh`, tag the image, and push it to Dockerhub:
  + ```
  + ~/envoy/ci/build_container $ DOCKER_USERNAME=user DOCKER_PASSWORD=pass ./update_build_container.sh
  + ```
  +3. After you have done that, update `ci/ci_steps.sh` in a new PR to pull the new tagged version of `lyft/envoy-build` during CI runs. Any PRs that depend on this image change will have to merge master after the change to `ci/ci_steps.sh` has been merged to master.
  