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
One option which worked for me on MAC.
Click on the Docker Icon in the tray. Open Preferences -> Proxies. Click on Manual Proxy and specify Web Server (HTTP) proxy and Secure Web server (HTTPS) proxy in the same format as we specify in HTTPS_PROXY env variable. Choose Apply and Restart.
This Worked for me