Dockerfile: allow specifying arch for base image

3 jobs for build-for-x86 in 24 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #421
s390x docker-alpine
build-s390x

00:00:23

passed #423
x86 docker-alpine
build-x86

00:00:17

passed #422
x86_64 docker-alpine
build-x86_64

00:00:18