Added container-based CFEngine package builder

Introduced build-in-container, a Python/Docker-based build system that
builds CFEngine packages inside containers using the existing build
scripts. Supports incremental builds via named Docker volumes, interactive
configuration with persistent defaults, and per-platform dependency caching.

Ticket: ENT-13777
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi