Temporary failure in name resolution [Errno -3] with Docker

I'm following the [docker tutorial][1] and am on the part where I have to build the app using: docker build -t friendlyhello . It reaches up to step 4, where after a pause I get this error: Step 4/7 : RUN pip install -r requirements.txt ---> Running in 7f4635a7510a Collecting Flask (from -r requirements.txt (line 1)) Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7fe3984d9b10>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/ I'm not quite sure what this error means and how I can go about solving it. Thanks for your help! [1]: https://docs.docker.com/get-started/part2/