Customize init command with optional arguments

When running `kamal init` a `deploy.yml` file with hard-coded values for repository, user, etc. is created. Users have to modify this file to suit their needs manually.

```yml
service: my-app # <<<

image: user/my-app # <<<

servers:
- 192.168.0.1 # <<<

registry:
username: my-user # <<<
```

## Proposition

These hard-coded values could either be inferred from the environment (e.g. username, directory...) or at least be customizable in the `kamal init` command. For example:

```bash
kamal init --service app-name --user user-name --server 192.168.0.1
```

I'd be happy to work on this. What do you think?

Start with the `kamal init` command and inspect how it creates `deploy.yml` with the current hard-coded values. Define the supported optional arguments or environment inference, then verify that the generated file uses the supplied service, user, and server values.

Written by the indexing model from the issue text.