At first I did set http://mirror.vutbr.cz/fedora/releases/25/Everything/x86_64/os/ as repo URL in below config.yaml.

config.yaml

name: ruby
packages:
    rpms:
        - ruby
...
module:
    rpm:
        repos:
            - http://mirror.vutbr.cz/fedora/releases/25/Everything/x86_64/os/

And did run below command.

sudo MODULE=nspawn avocado run ./*.py

And maybe /etc/yum.repos.d/ruby.repo was created in the process of above command.

/etc/yum.repos.d/ruby.repo

[ruby1]
name=ruby1
baseurl=http://mirror.vutbr.cz/fedora/releases/25/Everything/x86_64/os/
enabled=0
gpgcheck=0

Later I changed URL to https://mirrors.fedoraproject.org/metalink?repo=rawhide&arch=x86_64
.

And did run again.

sudo MODULE=nspawn avocado run ./*.py

But /etc/yum.repos.d/ruby.repo was not updated.
As a result, I got error for previous repo URL.
As this repo file created logic is hidden by the program, I wish the repo file is updated by update config.yaml.