Dockerfile: allow specifying arch for base image

3 jobs for build-for-x86 in 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #417
s390x docker-alpine
build-s390x

00:00:27

passed #419
x86 docker-alpine
build-x86

00:00:19

passed #418
x86_64 docker-alpine
build-x86_64

00:00:32