Eliminate or document the env var WARNINGs during start-proj.sh -l

[MikeTheCanuck]

When running start-proj.sh -l, I notice that four of our env vars aren't set, even though I followed our DOCKER-readme.md instructions. Here's what shows up at the beginning of the logged output when running the script:

$ ./budget_proj/bin/start-proj.sh  -l
Running start-proj.sh...
WARNING: The AWS_ACCESS_KEY_ID variable is not set. Defaulting to a blank string.
WARNING: The AWS_SECRET_ACCESS_KEY variable is not set. Defaulting to a blank string.
WARNING: The CONFIG_BUCKET variable is not set. Defaulting to a blank string.
WARNING: The DEPLOY_TARGET variable is not set. Defaulting to a blank string.

It's been a while since I tried the Docker build process for this project, so I don't recall if this was a necessary or spurious issue when using start-proj.sh -l, or if this is a problem we never got around to addressing.

If this is a spurious byproduct of using the same start-proj.sh script (with just the -l or -t flag to distinguish), then let's document this as a heads-up NOTE in the DOCKER-readme.md.

If this isn't necessary, but was just a byproduct of us not wanting to too-heavily uniqueify the docker-compose.yml files (local- and travis-), then perhaps it's time to comment these out of the local- version).