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

I'm following the docker tutorial 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!

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