build(iso): switch default Debian mirror, add --mirror override

mirror.mephi.ru was timing out during lb binary_rootfs. Default to
deb.debian.org (Fastly-backed, geo-balanced) and allow overriding via
--mirror on build-in-container.sh/build.sh, or BEE_MIRROR in auto/config.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

iso/builder/build-in-container.sh

@@ -12,6 +12,7 @@ CACHE_DIR="${BEE_BUILDER_CACHE_DIR:-${REPO_ROOT}/dist/cache}"
 AUTH_KEYS=""
 CLEAN_CACHE=0
 VARIANT="all"
+MIRROR=""
 
 . "${BUILDER_DIR}/VERSIONS"
 
@@ -33,9 +34,13 @@ while [ $# -gt 0 ]; do
             VARIANT="$2"
             shift 2
             ;;
+        --mirror)
+            MIRROR="$2"
+            shift 2
+            ;;
         *)
             echo "unknown arg: $1" >&2
-            echo "usage: $0 [--cache-dir /path] [--clean-build] [--authorized-keys /path/to/authorized_keys] [--variant nvidia|nvidia-legacy|amd|nogpu|all]" >&2
+            echo "usage: $0 [--cache-dir /path] [--clean-build] [--authorized-keys /path/to/authorized_keys] [--variant nvidia|nvidia-legacy|amd|nogpu|all] [--mirror http://host/debian/]" >&2
             exit 1
             ;;
     esac
@@ -156,10 +161,12 @@ run_variant() {
             -e TMPDIR=/cache/tmp \
             -e BEE_CACHE_DIR=/cache/bee \
             -e BEE_REQUIRE_MEMTEST=1 \
+            ${MIRROR:+-e "BEE_MIRROR=${MIRROR}"} \
             -w /work \
             "${IMAGE_REF}" \
             sh /work/iso/builder/build.sh --variant "${_v}" \
-                --authorized-keys "/tmp/bee-authkeys/${AUTH_KEYS_BASE}"
+                --authorized-keys "/tmp/bee-authkeys/${AUTH_KEYS_BASE}" \
+                ${MIRROR:+--mirror "${MIRROR}"}
     else
         "$CONTAINER_TOOL" run --rm --privileged \
             --platform "${BUILDER_PLATFORM}" \
@@ -171,9 +178,11 @@ run_variant() {
             -e TMPDIR=/cache/tmp \
             -e BEE_CACHE_DIR=/cache/bee \
             -e BEE_REQUIRE_MEMTEST=1 \
+            ${MIRROR:+-e "BEE_MIRROR=${MIRROR}"} \
             -w /work \
             "${IMAGE_REF}" \
-            sh /work/iso/builder/build.sh --variant "${_v}"
+            sh /work/iso/builder/build.sh --variant "${_v}" \
+                ${MIRROR:+--mirror "${MIRROR}"}
     fi
 }