How to fix config "myconfig" is not defined in Docker

The most common fix is to define the config at the top level of your docker-compose.yml. The service references a config name, and that name must be defined in the configs section:

Before (broken):

services:
  app:
    image: myapp
    configs:
      - myconfig  # References undefined config

After (fixed):

services:
  app:
    image: myapp
    configs:
      - myconfig

configs:
  myconfig:
    file: ./config/app.conf  # Path to local config file

The file attribute tells Docker Compose where to find the config content on your host machine.

Check that the config name in your service matches the top-level definition exactly, including case:

services:
  web:
    configs:
      - source: nginx_config  # Must match exactly
        target: /etc/nginx/nginx.conf

configs:
  nginx_config:  # This name must match the source above
    file: ./nginx.conf

Common mistakes:
- nginx-config vs nginx_config (hyphens vs underscores)
- NginxConfig vs nginxconfig (case differences)
- Extra whitespace in YAML

If you're using external: true, you must be in Swarm mode and create the config first:

# Initialize swarm mode (if not already done)
docker swarm init

# Create the config from a file
docker config create myconfig ./path/to/config.file

# Verify it was created
docker config ls

Then in your compose file:

configs:
  myconfig:
    external: true

Now docker stack deploy will find the external config.

If you're running docker compose up (not docker stack deploy), avoid external configs and use file-based configs instead:

configs:
  app_config:
    file: ./configs/application.yml

services:
  app:
    image: myapp
    configs:
      - source: app_config
        target: /app/config.yml
        mode: 0440

This approach works without Swarm mode in Docker Compose v2+.

Older docker-compose (Python-based v1) would silently ignore the configs section outside Swarm mode. Docker Compose v2 (Go-based, docker compose without hyphen) has better support:

# Check your version
docker compose version

# If you see v1.x, upgrade to v2
# On Ubuntu/Debian:
sudo apt-get update
sudo apt-get install docker-compose-plugin

# On macOS with Docker Desktop, update Docker Desktop
# Configs support was added around v2.0

After upgrading, use docker compose (with space) instead of docker-compose (with hyphen).

If configs aren't working in your environment, bind mounts achieve similar results:

services:
  app:
    image: myapp
    volumes:
      - ./config/app.conf:/etc/app/app.conf:ro

The :ro flag makes it read-only, similar to how configs work. This approach:
- Works in all Docker versions
- Doesn't require Swarm mode
- Changes to the file on the host are immediately visible in the container