Ignore stderr returned from docker-compose command

[Sam-Kramer]

What happened?

A docker-compose file we had was using a deprecated property, and as a result would warn log to stderr. This ends up breaking docker-compose-rule downstream whenever we parse command output (such as when we list services) as the warn message will be seen as a valid input.

What did you want to happen?

Separate stdin from stderr when parsing command responses; probably makes sense to not ignore the warn (maybe log) but shouldn't cause catastrophic failure.