europa: up: support --output

Support `dagger up --output <file.json>` or `-` for stdout. This will
write the computed layer.

Can be used for tests, e.g.

```
run dagger up --output -
assert_output --partial foobar
```

Fixes #1220

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

cmd/dagger/cmd/up.go

@@ -3,6 +3,7 @@ package cmd
 import (
 	"context"
 	"errors"
+	"fmt"
 	"os"
 
 	"cuelang.org/go/cue"
@@ -121,10 +122,23 @@ func europaUp(ctx context.Context, cl *client.Client, args ...string) error {
 	}
 
 	return cl.Do(ctx, p.Context(), func(ctx context.Context, s solver.Solver) error {
-		if err := p.Up(ctx, s); err != nil {
+		computed, err := p.Up(ctx, s)
+		if err != nil {
 			return err
 		}
 
+		if output := viper.GetString("output"); output != "" {
+			data := computed.JSON().PrettyString()
+			if output == "-" {
+				fmt.Println(data)
+				return nil
+			}
+			err := os.WriteFile(output, []byte(data), 0600)
+			if err != nil {
+				lg.Fatal().Err(err).Str("path", output).Msg("failed to write output")
+			}
+		}
+
 		return nil
 	})
 }
@@ -179,6 +193,7 @@ func checkInputs(ctx context.Context, env *environment.Environment) error {
 
 func init() {
 	upCmd.Flags().BoolP("force", "f", false, "Force up, disable inputs check")
+	upCmd.Flags().String("output", "", "Write computed output. Prints on stdout if set to-")
 
 	if err := viper.BindPFlags(upCmd.Flags()); err != nil {
 		panic(err)