osbuild: add skopeo-index source

New osbuild source that can download a manifest-list from a container
registry, using the `--multi-arch=index-only` option of skopeo copy.

internal/osbuild/skopeo_index_source.go

@@ -0,0 +1,57 @@
+package osbuild
+
+import (
+	"fmt"
+)
+
+type SkopeoIndexSource struct {
+	Items map[string]SkopeoIndexSourceItem `json:"items"`
+}
+
+func (SkopeoIndexSource) isSource() {}
+
+type SkopeoIndexSourceImage struct {
+	Name      string `json:"name"`
+	TLSVerify *bool  `json:"tls-verify,omitempty"`
+}
+
+type SkopeoIndexSourceItem struct {
+	Image SkopeoIndexSourceImage `json:"image"`
+}
+
+func (item SkopeoIndexSourceItem) validate() error {
+
+	if item.Image.Name == "" {
+		return fmt.Errorf("source item has empty name")
+	}
+
+	return nil
+}
+
+// NewSkopeoIndexSource creates a new and empty SkopeoIndexSource
+func NewSkopeoIndexSource() *SkopeoIndexSource {
+	return &SkopeoIndexSource{
+		Items: make(map[string]SkopeoIndexSourceItem),
+	}
+}
+
+// AddItem adds a source item to the source; will panic
+// if any of the supplied options are invalid or missing
+func (source *SkopeoIndexSource) AddItem(name, image string, tlsVerify *bool) {
+	item := SkopeoIndexSourceItem{
+		Image: SkopeoIndexSourceImage{
+			Name:      name,
+			TLSVerify: tlsVerify,
+		},
+	}
+
+	if err := item.validate(); err != nil {
+		panic(err)
+	}
+
+	if !skopeoDigestPattern.MatchString(image) {
+		panic("item has invalid image id")
+	}
+
+	source.Items[image] = item
+}