Add instructions to include custom files in the build env

[gthvn1]

No description provided.

[glehmann]

The user repositories are already installed by init-container.sh (

xcp-ng-build-env/files/init-container.sh

Lines 27 to 30 in 14eb61e

	if [ -n "$XCPREL" ]; then
	curl -s https://koji.xcp-ng.org/repos/user/${XCPREL}/xcpng-users.repo |
	sed '/^gpgkey=/ ipriority=1' | sudo tee /etc/yum.repos.d/xcp-ng-users.repo > /dev/null
	fi

) so this may not be the best example 🙂

[stormi]

Added Gaëtan as reviewer because he has a strong point.

[ydirson]

If we're moving towards publishing the build-env in an image registry, this approach for adding files does not look like a long-term one.
Some use-cases can likely be addressed when launching the container, by bind-mounting the file into the image, which for the example given would be something like -v /tmp/xcpng-users.repo:/etc/yum.repos.d/xcpng-users.repo