Add Hint: Enclose bool variables in quotes when using dictionary

[marb-dev]

Environment variables like true and false should be enclosed in quotes when using a dictionary; otherwise, their value will be parsed into True or False in the container, which makes them harder to be used within scripts.
Covers #1788

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "hint-quote-vars" git@github.com:marb-dev/compose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Ammending updates the existing PR. You DO NOT need to open a new one.

[aanand]

It absolutely sucks that Compose does this. Instead of telling users to work around it, we should treat it as a bug and fix it. There's already an issue for that - #1788 - and we're making it a priority for Compose 1.5.

As such, I'm going to close this PR. Thank you for contributing!