set ARCH to hardcoded arch

Dockerfile.aarch64

@@ -8,7 +8,7 @@ ARG S6_ARCH_aarch64=${S6_ARCH_aarch64}
 ARG S6_OVERLAY_VERSION=${S6_OVERLAY_VERSION}
 ARG ROOTFS=/root-out
 ARG REL=${APP_VERSION}
-ARG ALPINE_ARCH=aarch64
+ARG ARCH=aarch64
 ARG MIRROR=http://dl-cdn.alpinelinux.org/alpine
 ARG PACKAGES=alpine-baselayout,\
 alpine-keys,\