docker-image: Add option to disable arches

Not all images will be able to build on all arches. Use the 'except'
feature to exclude arches when a specific variable is set.

Fixes #1
1 job for optional-arches in 33 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Verify
passed #16972
docker-alpine
lint

00:00:33