runtime: support for context imports

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

plan/task/import.go

@@ -0,0 +1,37 @@
+package task
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"go.dagger.io/dagger/compiler"
+	"go.dagger.io/dagger/plancontext"
+	"go.dagger.io/dagger/solver"
+)
+
+func init() {
+	Register("Import", func() Task { return &importTask{} })
+}
+
+type importTask struct {
+}
+
+func (c importTask) Run(ctx context.Context, pctx *plancontext.Context, _ solver.Solver, v *compiler.Value) (*compiler.Value, error) {
+	var dir *plancontext.Directory
+
+	if err := v.Decode(&dir); err != nil {
+		return nil, err
+	}
+
+	// Check that directory exists
+	if _, err := os.Stat(dir.Path); os.IsNotExist(err) {
+		return nil, fmt.Errorf("%q dir doesn't exist", dir.Path)
+	}
+
+	id := pctx.Directories.Register(dir)
+	return compiler.Compile("", fmt.Sprintf(
+		`fs: #up: [{do: "local", id: %q}]`,
+		id,
+	))
+}