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/build-in-container.sh

@@ -9,6 +9,7 @@ CONTAINER_TOOL="${CONTAINER_TOOL:-docker}"
 IMAGE_TAG="${BEE_BUILDER_IMAGE:-bee-iso-builder}"
 BUILDER_PLATFORM="${BEE_BUILDER_PLATFORM:-linux/amd64}"
 CACHE_DIR="${BEE_BUILDER_CACHE_DIR:-${REPO_ROOT}/dist/container-cache}"
+APT_PROXY="${APT_PROXY:-}"
 AUTH_KEYS=""
 REBUILD_IMAGE=0
 
@@ -28,9 +29,13 @@ while [ $# -gt 0 ]; do
             AUTH_KEYS="$2"
             shift 2
             ;;
+        --apt-proxy)
+            APT_PROXY="$2"
+            shift 2
+            ;;
         *)
             echo "unknown arg: $1" >&2
-            echo "usage: $0 [--cache-dir /path] [--rebuild-image] [--authorized-keys /path/to/authorized_keys]" >&2
+            echo "usage: $0 [--cache-dir /path] [--rebuild-image] [--authorized-keys /path/to/authorized_keys] [--apt-proxy http://host:3142]" >&2
             exit 1
             ;;
     esac
@@ -100,6 +105,7 @@ set -- \
     -e GOMODCACHE=/cache/go-mod \
     -e TMPDIR=/cache/tmp \
     -e BEE_CACHE_DIR=/cache/bee \
+    ${APT_PROXY:+-e BEE_APT_PROXY="${APT_PROXY}"} \
     -w /work \
     "${IMAGE_REF}" \
     sh /work/iso/builder/build.sh
@@ -115,6 +121,7 @@ if [ -n "$AUTH_KEYS" ]; then
         -e GOMODCACHE=/cache/go-mod \
         -e TMPDIR=/cache/tmp \
         -e BEE_CACHE_DIR=/cache/bee \
+        ${APT_PROXY:+-e BEE_APT_PROXY="${APT_PROXY}"} \
         -w /work \
         "${IMAGE_REF}" \
         sh /work/iso/builder/build.sh --authorized-keys "/tmp/bee-authkeys/${AUTH_KEYS_BASE}"