Error when running PIP command in Docker

I have a python project that runs inside a Docker.

When the (Build) image is assembled reliably from a

RUN pip install -r requirements.txt 

but when it executes the command it returns a connection error.

Complete Dockerfile

FROM python:3.5

ADD ./app /app
WORKDIR /app
EXPOSE 5000
RUN pip install -r requirements.txt 
CMD ["python", "app.py"]

Error:

docker-compose build

Building socket
Step 1/6 : FROM python:3.5
 ---> e649f67bb435
Step 2/6 : ADD ./app /app
 ---> 92af2870125a
Step 3/6 : WORKDIR /app
Removing intermediate container e08611d47e59
 ---> cfa7b0c4904f
Step 4/6 : EXPOSE 5000
 ---> Running in c84d2c2adc31
Removing intermediate container c84d2c2adc31
 ---> 8d23ccfe53ff
Step 5/6 : RUN pip install -r requirements.txt
 ---> Running in 8f02d2e43eee
Collecting Flask==1.0.0 (from -r requirements.txt (line 1))
  Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742caec50>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742caeac8>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742cae908>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742caecf8>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742cae668>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Could not find a version that satisfies the requirement Flask==1.0.0 (from -r requirements.txt (line 1)) (from versions: )
No matching distribution found for Flask==1.0.0 (from -r requirements.txt (line 1))
ERROR: Service 'socket' failed to build: The command '/bin/sh -c pip install -r requirements.txt' returned a non-zero code: 1
Makefile:2: recipe for target 'build' failed
make: *** [build] Error 1ne)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742caeac8>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742cae908>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742caecf8>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f5742cae668>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/flask/
  Could not find a version that satisfies the requirement Flask==1.0.0 (from -r requirements.txt (line 1)) (from versions: )
No matching distribution found for Flask==1.0.0 (from -r requirements.txt (line 1))
ERROR: Service 'socket' failed to build: The command '/bin/sh -c pip install -r requirements.txt' returned a non-zero code: 1
Makefile:2: recipe for target 'build' failed
make: *** [build] Error 1