Support for runArgs for `docker compose` scenario

^{Originally posted by **bgloss** March 10, 2023}
I don't get the environment variables from my project's `.env` file into the `docker-compose.yml`setup with a pretty default project layout.

My project layout is as follows:

```
.devcontainer/
devcontainer.json
docker-compose.yml
Dockerfile
.env
```

I want to have in `docker-compose.yml` a section to create a Postgres container with setting up the environment one by one: `POSTGRES_USER`, `POSTGRES_PASSWORD`, and `POSTGRES_DB` and fill them with variables from the project's `.env` file. The relevant section in `docker-compose.yml` is:
```
postgres:
image: postgres:latest
environment:
POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
POSTGRES_USER: ${POSTGRES_USER}
POSTGRES_DB: ${POSTGRES_DB}
```

I do NOT want to have a copy of my `.env` file in the `.devcontainer/` folder, as it is the case in some of the DevContainer templates. This would work, as in this case `docker compose` loads it by default. I don't want this organization, as the `.env` file is also needed by the project's code and expected in the project root.

I also don't want to expose the whole `.env`-file to my container, which I could do with the `env_file` option in the docker compose service description.

When building a container myself (just `docker`, no `docker compose`), I can add in `devcontainer.json` the option `runArgs` next to the option `build` giving the env-file information to docker: `--env-file ../.env` However, this is not possible when using a docker-compose setup in `vscode` and `DevContainers` with `"dockerComposeFile": "docker-compose.yml"`.

https://containers.dev/implementors/json_reference/

At the command line `docker compose --env-file ../.env up` does the trick.

So, my questions:
- Am I missing something?
- Is this a missing feature in `devcontainer.json`?