can not build docker image can not connection pip server

[shencan]

`
Step 9/10 : RUN pip3 install -r requirements.txt
---> Running in 0c2795ada0bf
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 0x7f792376fbe0>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/flask/

Retrying (Retry(total=3, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f791eb602b0>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/flask/

Retrying (Retry(total=2, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f791eb60208>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/flask/

Retrying (Retry(total=1, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f791eb60c88>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/flask/

Retrying (Retry(total=0, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f791eb60ef0>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/flask/

Could not find a version that satisfies the requirement flask (from -r requirements.txt (line 1)) (from versions: )

�No matching distribution found for flask (from -r requirements.txt (line 1))
�mtime="2019-11-06T09:34:39Z" level=fatal msg="build failed: build failed: building [can.shen/rsvpapp]: build artifact: unable to stream build output: The command '/bin/sh -c pip3 install -r requirements.txt' returned a non-zero code: 1"

`