Suppress perl/apt locale warnings by configuring C.UTF-8 for apt-based systems. The script now exports DEBIAN_FRONTEND=noninteractive and LC_ALL/LANG/LANGUAGE=C.UTF-8 when PKG_MANAGER==apt, and appends LC_ALL and LANG to /etc/environment if not present. This addresses cases where a container inherits an ungenerated host locale (e.g. Devuan) and ensures C.UTF-8, which is always available, is used.
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