Initial pass at engine.#Scratch - failing

Signed-off-by: Joel Longtine <joel@dagger.io>

plan/task/scratch.go

@@ -0,0 +1,31 @@
+package task
+
+import (
+	"context"
+
+	"github.com/moby/buildkit/client/llb"
+	"go.dagger.io/dagger/compiler"
+	"go.dagger.io/dagger/plancontext"
+	"go.dagger.io/dagger/solver"
+)
+
+func init() {
+	Register("Scratch", func() Task { return &scratchTask{} })
+}
+
+type scratchTask struct {
+}
+
+func (t *scratchTask) Run(ctx context.Context, pctx *plancontext.Context, s solver.Solver, v *compiler.Value) (*compiler.Value, error) {
+	st := llb.Scratch()
+	result, err := s.Solve(ctx, st, pctx.Platform.Get())
+	if err != nil {
+		return nil, err
+	}
+
+	fs := pctx.FS.New(result)
+
+	return compiler.NewValue().FillFields(map[string]interface{}{
+		"output": fs.MarshalCUE(),
+	})
+}