devcontainer: support docker-compose

Devcontainer spec supports specifying a compose file

e.g.

```
"dockerComposeFile": "docker-compose.yml",
```

Example: https://github.com/BrunoQuaresma/openticket/blob/main/.devcontainer/devcontainer.json

Start by locating the code that reads the devcontainer specification and builds environments from a Dockerfile. Check how the dockerComposeFile field is currently handled, then compare the behavior with the referenced devcontainer example. Done means a compose file can be specified and used to build the development environment, with relevant tests updated or added.

Written by the indexing model from the issue text.