feat(iso): add APT_PROXY support to speed up builds via apt-cacher-ng

Pass APT_PROXY=http://host:3142 to build-in-container.sh to route
all apt traffic through a local cache. Also supports --apt-proxy flag.
Mirrors in auto/config are set from BEE_APT_PROXY env when present.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/auto/config

@@ -16,6 +16,14 @@ else
     LB_LINUX_PACKAGES="linux-image"
 fi
 
+# Route apt through proxy if BEE_APT_PROXY is set (e.g. apt-cacher-ng).
+# Usage: APT_PROXY=http://host:3142 ./build-in-container.sh
+if [ -n "${BEE_APT_PROXY:-}" ]; then
+    _MIRROR="${BEE_APT_PROXY%/}/debian"
+else
+    _MIRROR="https://deb.debian.org/debian"
+fi
+
 lb config noauto \
     --distribution bookworm \
     --architectures amd64 \
@@ -23,9 +31,9 @@ lb config noauto \
     --bootloaders "grub-efi,syslinux" \
     --debian-installer none \
     --archive-areas "main contrib non-free non-free-firmware" \
-    --mirror-bootstrap "https://deb.debian.org/debian" \
-    --mirror-chroot "https://deb.debian.org/debian" \
-    --mirror-binary "https://deb.debian.org/debian" \
+    --mirror-bootstrap "${_MIRROR}" \
+    --mirror-chroot "${_MIRROR}" \
+    --mirror-binary "${_MIRROR}" \
     --security true \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \