Precompile with defer, invalid Dockerfile

Adding the parameter ‘--precompile=defer’ an invalid Dockerfile is created.

```bash
$ rails generate dockerfile --precompile=defer
...
Dockerfile:27
--------------------
25 |
26 |
27 | >>> FROM prebuild as node
28 |
29 | # Install JavaScript dependencies
--------------------
ERROR: failed to solve: prebuild: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed
```

I changed line 5 and append "as prebuild" to fix it.

```yaml
FROM ruby:$RUBY_VERSION-slim as prebuild
```