changed ARCH to x86_64 in Dockerfile

Dockerfile

@@ -7,7 +7,7 @@ ARG APP_VERSION=${BUILD_TAG}
 ARG S6_OVERLAY_VERSION=${S6_OVERLAY_VERSION}
 ARG ROOTFS=/root-out
 ARG REL=${APP_VERSION}
-ARG ARCH=amd64
+ARG ARCH=x86_64
 ARG MIRROR=http://dl-cdn.alpinelinux.org/alpine
 ARG PACKAGES=alpine-baselayout,\
 alpine-keys,\