better error messages with docker api

[chrislovecnm]

Got `DOCKER_HOST="unix:///var/run/docker.sock" running on my mac. I do not have the time to try to figure out how to make socket connection, but the error message will help a user.

[justinsb]

Looks like docker changed the default, it is now https://github.com/docker/engine-api/blob/master/client/client_unix.go but in the version we're using Darwin is

// DefaultDockerHost defines os specific default if DOCKER_HOST is unset
const DefaultDockerHost = "tcp://127.0.0.1:2375"

(unix is DefaultDockerHost = "unix:///var/run/docker.sock")

Probably going to be best to use the Docker CLI throughout :-(

[chrislovecnm]

Would be better in mind to use the socket, I would rather now have to have the binary in a k8s hosted kops. Just a thought

[justinsb]

Yes, I definitely favor the socket as well. Except that using the socket means parsing the auth values ourselves, which is a pain. So we end up using the CLI for push & pull anyway. I'm thinking we just use the CLI everywhere, for now...

[justinsb]

Although this is an improvement on the API path, whether or not we want that long term :-)

/lgtm

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