Step 5/48 : RUN yum install -y gcc gcc-c++
 ---> Running in f810ab90a354
Loaded plugins: fastestmirror


 One of the configured repositories failed (Unknown),
 and yum doesn't have enough cached data to continue. At this point the only
 safe thing yum can do is fail. There are a few ways to work "fix" this:

     1. Contact the upstream for the repository and get them to fix the problem.

     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=<repoid> ...

     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 <repoid>
        or
            subscription-manager repos --disable=<repoid>

     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=<repoid>.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=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"

The command '/bin/sh -c yum install -y gcc gcc-c++' returned a non-zero code: 1

我的Docker是非联网状态的，所以不能从网络的wget下载包，在Docker里想把/dev/cdrom的目录挂载到/mnt/cdrom上，这样就能在Docker里使用本地的yum源了，但在Docker里执行mount命令失败。

现请问怎么能在Dockerfile里使用本地的yum源来执行yum命令？

如果非得要用网络的yum源，请问怎么检查Docker容器是否能联网？