Docker在build自定义镜像时 报错 ：WARNING：IPv4 forwarding is disabled. Networking will not work. 解决办法

. 首先给出解决办法，再说解决过程中遇到的坑

1. 第一步：
   vi /etc/sysctl.conf
   net.ipv4.ip_forward=1 #添加这段代码
2. 第二步
   #重启network服务
   systemctl restart network && systemctl restart docker

坑来了：
报错L:
*

WARNING： IPv4 forwarding is disabled. Networking will not work.
2. Reconfigure the baseurl/etc. for the repository, to point to a working
        upstream. This is most often useful if you are using a newer
        distribution release than is supported by the repository (and the
        packages for the previous distribution release still work).
     3. Run the command with the repository temporarily disabled
            yum --disablerepo= ...
     4. Disable the repository permanently, so yum won't use it by default. Yum
        will then just ignore the repository until you permanently enable it
        again or use --enablerepo for temporary usage:
            yum-config-manager --disable 
        or
            subscription-manager repos --disable=
     5. Configure the failing repository to be skipped, if it is unavailable.
        Note that yum will try to contact the repo. when it runs most commands,
        so will have to try and fail each time (and thus. yum will be be much
        slower). If it is a very temporary problem though, this is often a nice
        compromise:
            yum-config-manager --save --setopt=.skip_if_unavailable=true
Cannot find a valid baseurl for repo: base/7/x86_64
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=container error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
The command '/bin/sh -c yum -y install vim' returned a non-zero code: 1*

网上找了半天，都一直以为是DNS配置出错，结果是 IPv4 forwarding is disabled