Provide build arguments for docker-compose in docker-compose.yml setups

Hi. There's an issue right now with VSCode on MacOS where environment variables are not loaded from the default system shell when VSCode is launched from the dock, likely because VSCode launches with posix_spawn() which doesn't use the system shell. https://github.com/microsoft/vscode/issues/118460

We're looking into using devcontainers for our development workflow as it brings a lot of benefits, the biggest being quick access to the container's shell during development directly through a VSCode terminal without having to run docker commands.

The problem is that we have submitted our .env file into our Git repository for our codebase because it provides some sane default build arguments for docker-compose that should be enabled for all developers. We have a separate workflow for storing secrets for our Docker image build arguments. Usually this means storing secrets in a file called .env.development.local. In a regular docker-compose set up this wouldn't be a problem as we can just provide a --env-file.

Unfortunately, we can't use the --env-file flag in the devcontainer build process because there's no way of specifying this in the devcontainer.json file.

Here's what the command looks like in my machine during a devcontainer setup:

[716 ms] Start: Run: docker-compose --project-name backend -f /Users/dimgl/dev/backend/docker-compose.yml -f /Users/dimgl/dev/backend/.devcontainer/docker-compose.yml build --no-cache --pull db redis app

Here we would put a --env-file .env.development.local file to send in secrets. So we'd like to be able to do something like this:

{
  "dockerComposeFile": [
    "../docker-compose.yml",
    "docker-compose.yml"
  ],
  "buildArgs": [
    "--env-file",
    ".env.development.local"
  ]
}

Thanks.