Ensures that we're running on top of the latest Debian release, compatibly with a schedule-based image regeneration approach.
Meant to prevent s6-telinit from erroring out when the container is started with another init (docker run --init). Fixes issue #1.
This reverts commit 3bc8d2b2. Breaks with: Starting s6-linux-init... s6-linux-init-telinit: usage: s6-linux-init-telinit runlevel