Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/solver/llbsolver/file/refmanager.go

@@ -0,0 +1,70 @@
+package file
+
+import (
+	"context"
+
+	"github.com/moby/buildkit/cache"
+	"github.com/moby/buildkit/snapshot"
+	"github.com/moby/buildkit/solver/llbsolver/ops/fileoptypes"
+	"github.com/pkg/errors"
+)
+
+func NewRefManager(cm cache.Manager) *RefManager {
+	return &RefManager{cm: cm}
+}
+
+type RefManager struct {
+	cm cache.Manager
+}
+
+func (rm *RefManager) Prepare(ctx context.Context, ref fileoptypes.Ref, readonly bool) (fileoptypes.Mount, error) {
+	ir, ok := ref.(cache.ImmutableRef)
+	if !ok && ref != nil {
+		return nil, errors.Errorf("invalid ref type: %T", ref)
+	}
+
+	if ir != nil && readonly {
+		m, err := ir.Mount(ctx, readonly)
+		if err != nil {
+			return nil, err
+		}
+		return &Mount{m: m}, nil
+	}
+
+	mr, err := rm.cm.New(ctx, ir, cache.WithDescription("fileop target"), cache.CachePolicyRetain)
+	if err != nil {
+		return nil, err
+	}
+	m, err := mr.Mount(ctx, readonly)
+	if err != nil {
+		return nil, err
+	}
+	return &Mount{m: m, mr: mr}, nil
+}
+
+func (rm *RefManager) Commit(ctx context.Context, mount fileoptypes.Mount) (fileoptypes.Ref, error) {
+	m, ok := mount.(*Mount)
+	if !ok {
+		return nil, errors.Errorf("invalid mount type %T", mount)
+	}
+	if m.mr == nil {
+		return nil, errors.Errorf("invalid mount without active ref for commit")
+	}
+	defer func() {
+		m.mr = nil
+	}()
+	return m.mr.Commit(ctx)
+}
+
+type Mount struct {
+	m  snapshot.Mountable
+	mr cache.MutableRef
+}
+
+func (m *Mount) Release(ctx context.Context) error {
+	if m.mr != nil {
+		return m.mr.Release(ctx)
+	}
+	return nil
+}
+func (m *Mount) IsFileOpMount() {}