Switch build container to debian
Switch build container to debian

Our calicoctl binary depends on libc, and so we will bake in
a dependency on a newer distro by using something modern like
Ubuntu 14.04.

Switch to debian 7, which is new enough to meet our requirements,
while adding support to older distros.

(debian 7 comes with libc version 2.13-38+deb7u6).


Former-commit-id: 8027c4dfd418919a8090dfa9ca162a56ff8f4cf5
TrimBiggs/libnetwork-plugin,projectcalico/libnetwork-plugin,tomdee/libnetwork-plugin,TrimBiggs/libnetwork-plugin