Provide a way to read stdout on non-zero exit code

[ovesh]

If a container exits non-zero, an ContainerError is thrown, but this object only provides access to stderr, not stdout. Some binaries write their logs/errors to stdout instead of stderr, or at least stdout can provide a clue on why the command failed.

[tristan-f-r]

This is a duplicate of https://github.com/docker/docker-py/issues/2745.