envbuilder don't recognize "dockerComposeFile" param

Hi! My devcontainer.json looks like this:

```json
{
"name": "keycrm",
"dockerComposeFile": [
"../example.docker-compose.yml"
],
"service": "app",
"services": ["app", "db"],
"forwardPorts": [80],
"postAttachCommand": "service apache2 start",
"extensions": [
"felixfbecker.php-pack",
"cweijan.vscode-mysql-client2"
]
}
```

and coder don't recognize "dockerComposeFile" option. It's described in devcontainer.json schema, and supported by my IDE (phpstorm) as well. But when I'm trying to create a workspace with such devcontainer it usually fallback to default image.
So, maybee in future, would envbuilder support docker-compose files to build, or I'm using it in wrong way? I'm trying to make developer's workspace with standalone environment, but creating only "app" is not enough in case of no DB.

提供された devcontainer.json と ../example.docker-compose.yml から始め、envbuilder が dockerComposeFile オプションをデフォルトのイメージパスとどのように扱うかを追跡します。app サービスと db サービスをビルドして起動すべきかを定義し、この設定を使用する workspace がデフォルトのイメージにフォールバックしなくなったことを検証します。

索引モデルが issue の本文から書いたものです。