解决Dockerfile运行报错：ERROR: failed to solve: process “/bin/sh -c pip3 install flask“

运行Dockerfile时报错，Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ConnectTimeoutError(<urllib3.connection.VerifiedHTTPSConnection object at 0x7f22bcb635f8>, 'Connection to pypi.python.org timed out. (connect timeout=15)')': /simple/flask/

出现该报错是因为pip install flask时用的国外的服务器，导致连接失败，因此只需要在Dockerfile文件的pip install flask后面跟上国内的镜像地址就行，如阿里云：

RUN pip3 install flask -i http://mirrors.aliyun.com/pypi/simple/ --trusted-host mirrors.aliyun.com