Debian has two Dockerfiles associated - minimal and regular one that
bases on the minimal one.

Let's mention them by name and add a comment providing some context to
the split so it's easier to figure out what to install on a new system.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>