Environment Variables in volumes trigger named volume warning

[karlkfi]

Warning added in #1833 is triggered by volumes starting with environment variables (e.g. ${VOLUME_DIR}:/var/stuff).

How would we know whether the env-var was a name or a path?

Is there a way to wait to test for .~/ until after env-vars are evaluated?

[aanand]

This is partly a bug, and partly a side-effect of how variable substitution has been implemented in the past for volume paths.

First of all, Compose is substituting variables before checking the host path. That's not the issue. However, two things are currently wrong:

1. The error message prints out the input string, rather than the evaluated string. That's an easy one to fix - I'll do that now.
2. The method we use to substitute variables - os.path.expandvars - performs no expansion if a variable is unset, meaning the final value will still be ${VOLUME_PATH}:/var/stuff. This is obviously (to me) a bug. The good news is that it's already fixed in master, as a result of the new universal variable substitution code merged in Interpolate environment variables #1765.

[karlkfi]

Thanks @aanand!