Use SLC6 container to build SLC5 binaries
I've been mentioning multiple times that we can build SLC5 binaries on SLC6, in particular discussing the possibility of using singularity instead of docker.
The only drawback of this change is that the produced binaries will not be usable on a real SLC5, but such a thing most probably doesn't exist anywhere on the Grid.
Edited by Marco Clemencic