engine: Support plan outputs

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

plan/task/outputdirectory.go

@@ -0,0 +1,43 @@
+package task
+
+import (
+	"context"
+
+	bk "github.com/moby/buildkit/client"
+	"go.dagger.io/dagger/compiler"
+	"go.dagger.io/dagger/plancontext"
+	"go.dagger.io/dagger/solver"
+)
+
+func init() {
+	Register("OutputDirectory", func() Task { return &outputDirectoryTask{} })
+}
+
+type outputDirectoryTask struct {
+}
+
+func (c outputDirectoryTask) Run(ctx context.Context, pctx *plancontext.Context, s solver.Solver, v *compiler.Value) (*compiler.Value, error) {
+	contents, err := pctx.FS.FromValue(v.Lookup("contents"))
+	if err != nil {
+		return nil, err
+	}
+
+	dest, err := v.Lookup("dest").String()
+	if err != nil {
+		return nil, err
+	}
+
+	st, err := contents.Result().ToState()
+	if err != nil {
+		return nil, err
+	}
+	_, err = s.Export(ctx, st, nil, bk.ExportEntry{
+		Type:      bk.ExporterLocal,
+		OutputDir: dest,
+	}, pctx.Platform.Get())
+	if err != nil {
+		return nil, err
+	}
+
+	return compiler.NewValue(), nil
+}