Add SLES 12 builds #57
Description
The SLES Base Container Images (BCI) require an active paid subscription to access the repositories, which we need in order to use them for builds.
https://documentation.suse.com/container/all/html/Container-guide/index.html#sec-bci-suseconnect
The only reason we are able to build SLES 15 is because they do make it available for free for that image, albeit unsupported.
https://registry.suse.com/repositories/suse-ltss-sle12-5-sles12sp5 notes "requires login" whereas https://registry.suse.com/repositories/suse-sle15sp6 does not.
In order to build SLES 12, we will likely need a subscription. @tuxmea is reaching out to his contacts to see what we can do here.