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.

Start with the supplied devcontainer.json and ../example.docker-compose.yml, then trace how envbuilder handles the dockerComposeFile option versus its default image path. Define whether the app and db services should be built and started, and verify that a workspace using this configuration no longer falls back to the default image.

Written by the indexing model from the issue text.