Docker client version determination fails when Docker daemon is not available about docker-credential-gcr

[dekkagaijin]

We've recently experienced the same issue with gcloud, as well, thanks for the report. As mentioned, configure-docker fails open, so this should only be an annoyance in practice. We check the version of the docker client for a couple reasons-
a) to verify that docker is visible on the path
b) to configure itself as a credential store for client versions 1.11 and 1.12.

[tmshn]

imo, all we need is client version, so we can use docker --version instead of docker version

[dekkagaijin]

Added more clarification to that warning, for cases like this: #45

I'm inclined to lean on helpful warning text more so than fancy error correction logic, since checking docker version can fail for any number of reasons which don't impact the function of the cred helper. Hopefully this updated error text will allow for users to verify that nothing's actually amiss...

[dekkagaijin]

configure-docker no longer checks the Docker client version:
https://github.com/GoogleCloudPlatform/docker-credential-gcr/releases/tag/v2.0.0