Compose: Suggesting escaping environment variables with double-dollar sign in error message

[nhooey]

Pull request #2204 documented the double-dollar sign ($$VARIABLE) escaping in the _documentation_, how about putting the same suggestion in the _error message_:

Change this:

WARNING: The VARIABLE variable is not set. Defaulting to a blank string.

To this:

WARNING: The VARIALBE variable is not set. Defaulting to a blank string.
         Did you forget to escape VARIABLE with two dollar signs like this: `$$VARIABLE`
         in your docker-compose.yml file?

Would make the error message much more useful and less misleading.

[shin-]

To me that seems very specific. There's many reasons why a variable might be unset. IMO that would be making too many assumptions.

[nhooey]

How about providing a list of possible reasons?

How many reasons can you think of where the user would be expecting the variables to be set, but they're actually not?

[max-vogler]

This is a great idea and would've saved me a lot of experiments with backslashes and a Google search.

[alancnet]

So uh... this is built-in now, right? I landed on this issue trying to figure out how to do it, and it works.