docker-image: explicitly provide DOCKER_ARCH for build jobs

We need to be able to provide both the docker arch naming standard as
the Alpine arch naming standard. When building on top of Alpine managed
images, the architecture name is different, so we need to be able to
specify that.
1 job for master in 34 seconds
Status Job ID Name Coverage
  Verify
passed #28733
docker-alpine
lint

00:00:34