Add a --token global flag

[silvin-lubecki]

A user activating the 2FA on his/her account makes the hub-tool ask for 6-digit code in a regularly basis. This behavior doesn't fit well with CI use cases. We should add a global flag like --token which accepts a Personal Access Token (with its current limitations) to bypass 2FA login. We may also support an env var DOCKER_HUB_PERSONAL_ACCESS_TOKEN.

[thaJeztah]

Will the token be passed on the command line, or through stdin?

wrt the env-var: we should come to a good name for that (and if we implement, use the same name for all our cli's), as it may be important to allow scanning for such env-vars being committed to source control etc

[zliang-akamai]

Something similar to docker's --password-stdin option would be very nice!