CI: build node images for amd64 as well as arm64 (node-26#28)

amd64 nodes (radio-box) got 'no matching manifest for linux/amd64' on
pull and had to build locally. Every image workflow now builds
linux/amd64,linux/arm64. Release images treats an existing src- tag as
present only if it carries both platforms, so the arm64-only src- tags
pushed before this get rebuilt on the next v1 push.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

install.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 #
-# DRB edge node — one-shot bootstrap for a clean Raspberry Pi OS (arm64).
+# DRB edge node — one-shot bootstrap for a clean Raspberry Pi OS (arm64) or Debian (amd64).
 #
 #   curl -fsSL https://git.vpn.cusano.net/logan/node-26/raw/tag/v1/install.sh \
 #     | sudo bash -s -- --node-id node-003 \
@@ -228,8 +228,8 @@ command -v apt-get >/dev/null || die "apt-get not found — this script targets
 
 ARCH="$(dpkg --print-architecture)"
 case "$ARCH" in
-  arm64|aarch64) ok "arch $ARCH" ;;
-  *) warn "arch is $ARCH — CI only builds linux/arm64 images. Prebuilt pull will fail; use --build." ;;
+  arm64|aarch64|amd64|x86_64) ok "arch $ARCH" ;;
+  *) warn "arch is $ARCH — CI only builds linux/amd64 + linux/arm64 images. Prebuilt pull will fail; use --build." ;;
 esac
 ok "running as root"