Set C.UTF-8 locale for apt-based containers

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.

misc/install.func

@@ -878,6 +878,16 @@ setting_up_container() {
   # Remove Python EXTERNALLY-MANAGED restriction (Debian 12+, Ubuntu 23.04+)
   rm -rf /usr/lib/python3.*/EXTERNALLY-MANAGED 2>/dev/null || true
 
+  # Fix locale warnings from perl/apt (e.g. Devuan inherits host locale but hasn't generated it)
+  # C.UTF-8 is always available without locale-gen on any Linux system
+  if [[ "$PKG_MANAGER" == "apt" ]]; then
+    export DEBIAN_FRONTEND=noninteractive
+    export LC_ALL=C.UTF-8
+    export LANG=C.UTF-8
+    export LANGUAGE=C.UTF-8
+    grep -qxF 'LC_ALL=C.UTF-8' /etc/environment 2>/dev/null || echo -e 'LC_ALL=C.UTF-8\nLANG=C.UTF-8' >>/etc/environment
+  fi
+
   # Disable network wait services for faster boot
   case "$INIT_SYSTEM" in
   systemd)