docker-image: use proper platform designations

The platform argument for docker build uses different names for arches
than the image prefix. For example, to pull an image for armv7, you need
to use `arm32v7/alpine`, but to build an image for that arch, you should
use `--platform arm`.

Use a separate environment variable to designating the docker platform.
1 job for master in 35 seconds
Status Job ID Name Coverage
  Verify
passed #43234
docker-alpine
lint

00:00:35