factor out repetitions in tags etc in docker-bake.hcl

[maxheld83]

this stuff could use some factoring out, maybe via the stdlib standard library:

function "compose_cache_from_field" {
  params = [image_name_stage]
  // should also cache from other registries, tracked in #219
  result = [
    "type=registry,ref=${REGISTRY_PREFIX_CI}/${IMAGE_OWNER}/${IMAGE_NAME_ROOT}/${image_name_stage}/cache/linux/arm64:${TAG_FROM_GIT_REF_NAME}",
    "type=registry,ref=${REGISTRY_PREFIX_CI}/${IMAGE_OWNER}/${IMAGE_NAME_ROOT}/${image_name_stage}/cache/linux/amd64:${TAG_FROM_GIT_REF_NAME}",
    "type=registry,ref=${REGISTRY_PREFIX_CI}/${IMAGE_OWNER}/${IMAGE_NAME_ROOT}/${image_name_stage}/cache/linux/arm64:${DEFAULT_BRANCH}",
    "type=registry,ref=${REGISTRY_PREFIX_CI}/${IMAGE_OWNER}/${IMAGE_NAME_ROOT}/${image_name_stage}/cache/linux/amd64:${DEFAULT_BRANCH}"
  ]
}