Add support for ARM64

This patch also removes the need for a separate ARM64 Dockerfile.
It has been tested on an Odroid-C2.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/build.sh

@@ -14,9 +14,8 @@ if [ "$arch" = "armv7l" ] ; then
    GOARM="7"
 elif [ "$arch" = "aarch64" ] ; then
    arch="arm64"
-   dockerfile="Dockerfile.arm64"
+   dockerfile="Dockerfile"
    eTAG="latest-arm64-dev"
-   GOARM="8"
 fi
 
 echo "$1"