add test script for arm docker build

docker/test/test-arm-build.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+set -e
+
+# register QEMU handlers for emulation (one-time)
+docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
+
+# create and use a buildx builder (if you don't already have one)
+if ! docker buildx inspect mybuilder > /dev/null 2>&1; then
+    docker buildx create --name mybuilder --use
+else
+    docker buildx use mybuilder
+fi
+docker buildx inspect --bootstrap
+
+# change to repo root
+cd ../..
+
+# build the alpine image for arm64 and amd64
+# Note: We cannot use --load with multiple platforms. 
+# We will build for arm64 specifically to test the emulation and load it.
+echo "Building for linux/arm64..."
+docker buildx build --platform linux/arm64 -f Dockerfile.alpine -t local-speedtest:alpine-arm64 --load .
+
+# To build for both platforms, you typically need to push to a registry:
+# docker buildx build --platform linux/amd64,linux/arm64 -f Dockerfile.alpine -t local-speedtest:alpine --push .