Docker image contains an old, now defunct source of SP packages.

This file in the Docker image is apparently unused because it points to an out of date source of SP packages, and the source URL is now finally dead.

/etc/yum.repos.d/security\:shibboleth.repo

I don't know how the SP is involved if at all in the build anymore, but it couldn't have been from there so removing the file should be a no-op.