Fix issue where $AWS_ENDPOINT_OPT is ignored

[sadortun]

when $AWS_ENDPOINT_URL present

[deitch]

This isn't 100% correct. As of now, AWS_ENDPOINT_OPT is not a published variable you can use. It is an internal var to functions.sh and entrypoint. That having been said, it does read it using file_env, so it implies it would be recognized. It sort of is between the fences.

This would make it into a recognized first-class var that can be set, which is a "good thing".

Let's add this to the README as documentation, and then it officially will become one, and we can merge it in.