Docker: container cannot find local repo - linux

Question

I am trying to build a centos image, then run basic yum commands from a company network with no internet access. After successfully grabbing the centos artifact in step 1, next comes RUN yum update where the container tries to load plugins using http://mirrorlist.centos.org, and that obviously will not work. It cannot resolve that host because no web access. So, I get the error:
Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
..."Could not resolve host http://mirrorlist.centos.org; Unknown error"
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
The command '/bin/sh -c yum update' returned a non-zero code: 1
I have a repo file in /etc/yum.repos.d that contains content described here. In that file, I have multiple local repo URLs. An [updates] entry has a baseurl for /updates. Is this entry supposed to be used by the container when I do a RUN yum update in my Dockerfile? How does the container know where to look for a local mirror repo or other repo?
Is there also an issue regarding localhost on the host vs. localhost in the container?
I have researched a dozen S.O. entries with no luck.
UPDATE: Dockerfile so far...
FROM path.to.repo/centos
RUN yum update
So, it errors upon yum update.

Answer 1

When you're creating images that can't reach the web but only internal network, you must change tools configuration before trying to use them.
With yum, you have to remove existing repos and replacing them with yours before RUN yum update, something like that :
FROM path.to.repo/centos
RUN rm -rf /etc/yum.repos.d/*.repo
COPY myprivate.repo /etc/yum.repos.d/
RUN yum update
File myprivate.repo must be defined in the same folder as your Dockerfile and must declares your repos.
Furthermore, this created image can now be used as a base image for all others images you need to create.