tests: Update alpine base to multi-arch

The following packages did not work on aarch64:
- linuxkit/test-docker-bench
- linuxkit/test-ltp
- linuxkit/test-ns
- linuxkit/test-virtsock

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

test/pkg/docker-bench/Dockerfile

@@ -1,4 +1,4 @@
-FROM linuxkit/alpine:34af9cb1990debd17fae6d4198c62ce3910d9908 AS mirror
+FROM linuxkit/alpine:87a0cd10449d72f374f950004467737dbf440630 AS mirror
 RUN mkdir -p /out/etc/apk && cp -r /etc/apk/* /out/etc/apk/
 RUN apk add --no-cache --initdb -p /out \
     alpine-baselayout \

test/pkg/docker-bench/Makefile

@@ -1,6 +1,7 @@
 IMAGE=test-docker-bench
 DEPS=bench_runner.sh
 NETWORK=1
+ARCHES=x86_64
 
 include ../../../pkg/package.mk