Suggest to use the obs:// prefix for adding OBS repos

[tomschr]

This PR contains a small change, but I think it gives us some benefits:

• shorter than with the https prefix
• you can avoid the Leap release; it's detected automatically by zypper
• better to maintain (as you do not have to take care of the Leap release)

What do you think? :slightly_smiling_face:

[ghost]

This is irrelevant now, as we're now building the container in OBS, where necessary repos are added automatically. (https://build.opensuse.org/package/view_file/Documentation:Containers/daps-toolchain/Dockerfile?expand=1)