I am having this issue in my Mac system 10.11.6
system3:postgres saurabh-gupta2$ docker build -t postgres . Sending build context to Docker daemon 38.91kB Step
Just to add, in case anyone else comes across this issue.
On a Mac I had to logout and log back in.
docker logout docker login
Then it prompts for username (NOTE: Not email) and password. (Need an account on https://hub.docker.com to pull images down)
Then it worked for me.