container: add new resolver helper

Add a new class `container.Resolver` which can be used to resolve
multiple container images to their respective ids in parallel.
It should make it easy for all existing tools and api endpoints
to adpot container resultion.

internal/container/resolver.go

@@ -0,0 +1,73 @@
+package container
+
+import (
+	"context"
+	"fmt"
+	"strings"
+)
+
+type resolveResult struct {
+	spec Spec
+	err  error
+}
+
+type Resolver struct {
+	jobs  int
+	queue chan resolveResult
+
+	ctx context.Context
+
+	Arch string
+}
+
+func NewResolver(arch string) Resolver {
+	return Resolver{
+		ctx:   context.Background(),
+		queue: make(chan resolveResult, 2),
+		Arch:  arch,
+	}
+}
+
+func (r *Resolver) Add(source, name string, TLSVerify *bool) {
+	client, err := NewClient(source)
+	r.jobs += 1
+
+	if err != nil {
+		r.queue <- resolveResult{err: err}
+		return
+	}
+
+	client.SetTLSVerify(TLSVerify)
+	client.SetArchitectureChoice(r.Arch)
+
+	go func() {
+		spec, err := client.Resolve(r.ctx, name)
+		if err != nil {
+			err = fmt.Errorf("'%s': %w", source, err)
+		}
+		r.queue <- resolveResult{spec: spec, err: err}
+	}()
+}
+
+func (r *Resolver) Finish() ([]Spec, error) {
+
+	specs := make([]Spec, 0, r.jobs)
+	errs := make([]string, 0, r.jobs)
+	for r.jobs > 0 {
+		result := <-r.queue
+		r.jobs -= 1
+
+		if result.err == nil {
+			specs = append(specs, result.spec)
+		} else {
+			errs = append(errs, result.err.Error())
+		}
+	}
+
+	if len(errs) > 0 {
+		detail := strings.Join(errs, "; ")
+		return specs, fmt.Errorf("failed to resolve container: %s", detail)
+	}
+
+	return specs, nil
+}