Export `LOCAL_WORKSPACE_FOLDER` env var to docker compose

On my devcontainers, I need to mount my workspace folder to the same location as they are in my host machine.

That's required because:

1. Otherwise bind mounts when running docker run inside of the devcontainer won't work
2. Otherwise docker-compose commands within the devcontainer won't work (because docker-compose identifies projects based on their folder names, and they would differ from host to inside the devcontainer - the latter usually is always workspace or so)

When I don't use docker-compose, the solution is simple:

// .devcontainer/devcontainer.json

{
  // Mounts the repository in the devcontainer in the same location of the repository in the host.
  // This is needed for running docker containers inside of the devcontainer when mounting volumes
  // inside of the repository, as docker in the devcontainer will talk to the docker daemon in the host.
  "workspaceMount": "source=${localWorkspaceFolder},target=${localWorkspaceFolder},type=bind,consistency=cached",
  "workspaceFolder": "${localWorkspaceFolder}",
}

But when using docker-compose, the only solution I found was a lot trickier:

.devcontainer/initialize.sh

#!/bin/bash

set -euo pipefail

readonly wanted_line_key="LOCAL_WORKSPACE_FOLDER"
readonly wanted_line="${wanted_line_key}='${PWD}'"
readonly file=".env"

echo "Writing ${wanted_line} to ${file}" >&2
if [[ -f "${file}" ]] && grep --quiet "^${wanted_line_key}=" "${file}"; then
    sed --in-place "s,^${wanted_line_key}=.*,${wanted_line}," "${file}"
else
    echo "${wanted_line}" >>"${file}"
fi

.devcontainer/devcontainer.json

{
  "dockerComposeFile": "../docker-compose.yml",
  "service": "dev",
  "workspaceFolder": "${localWorkspaceFolder}",
  "shutdownAction": "stopCompose",
  "initializeCommand": ".devcontainer/initialize.sh"
}

docker-compose.yml

version: '3'
services:
  dev:
    user: node
    build: .devcontainer
    env_file:
      - .env
    volumes:
      - .:${LOCAL_WORKSPACE_FOLDER}:cached

The above could be greatly simplified if VS Code could export the LOCAL_WORKSPACE_FOLDER before running docker-compose commands so that I could simply refer it there.