docker-compose.yaml inside .devcontainer can't access env vars from .env file outside during build

[Dima-Bulavenko]

HI, I'm trying to use a .env file located outside the .devcontainer/ folder to provide build arguments to docker-compose.yaml during a Dev Container setup.

However, since docker-compose.yaml is placed inside .devcontainer/, it doesn't pick up environment variables from the .env file in the parent directory during build-time. As a result, the args like ${PYTHON_VERSION} are empty when the image builds.

I’d like to avoid duplicating the .env file by copying it into .devcontainer/.

---

📁 Folder Structure

my-project/
├── .devcontainer/
│   ├── devcontainer.json
│   ├── docker-compose.yaml
│   ├── Dockerfile
├── .env  ← I want to use this file

---

📄 .env

PYTHON_VERSION=3.9
POETRY_VERSION=1.7.1
NODE_VERSION=20

---

📄 .devcontainer/devcontainer.json

{
  "dockerComposeFile": "docker-compose.yaml",
  "service": "development",
  "workspaceFolder": "/code",
  // I tried using initializeCommand to load the .env file,
  // but docker-compose still doesn't see the variables at build-time.
  "initializeCommand": "export $(grep -v '^#' .env | xargs) && echo ${PYTHON_VERSION}"
}

---

📄 .devcontainer/docker-compose.yaml

services:
  development:
    build:
      context: ../
      dockerfile: .devcontainer/Dockerfile
      args:
        # These should be set from the .env file, but they are actually empty
        - PYTHON_VERSION=${PYTHON_VERSION}
        - POETRY_VERSION=${POETRY_VERSION}
        - NODE_VERSION=${NODE_VERSION}
    env_file:
      - ../.env  # Works at runtime, not at build-time

---

🧪 Additional Note

When I placed docker-compose.yaml at the same level as the .env file, everything worked fine — the variables were picked up during build. But that’s not possible in my case because Dev Containers require docker-compose.yaml to be inside .devcontainer/.

---

❓ Question

How can I configure the Dev Container so that:

• docker-compose.yaml (inside .devcontainer/) uses a .env file from the parent folder
• The .env variables are correctly passed as build-time args
• I don’t have to duplicate the .env file inside .devcontainer/

Is there an officially supported way to handle this setup?

Thanks in advance!