How do I seed a Docker Compose media volume with initial data on startup?

When using named volumes (like your media volume), Docker Compose creates an empty volume on first run even if your image contains files in that directory. In other words, Docker doesn’t automatically populate a volume with the files that might have been added during the image build.

Below are two common approaches to “seed” your media volume when it’s empty:

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1. Use an Initialization Step in the Entrypoint

Modify your entrypoint (or add a new initialization script) to check whether the media directory is empty. If it is empty, copy the files from a directory in your image that contains the initial media data into the mounted volume.

Steps:

• Include the initial media files in your image:
  Remove the /media directory from your .dockerignore (or copy its contents to another directory inside the image, e.g. /init-media) so that you have the data available inside the container.

• Update your entrypoint:
  Add a check to see if the /app/media directory is empty. If it is, copy files from /init-media (or wherever the initial data is stored in the image) to /app/media.

Example Update to entrypoint.sh:

BASHCopied
#!/bin/sh
echo "Waiting for MySQL..."
while ! nc -z $MYSQL_HOST $MYSQL_PORT; do
  sleep 0.1
done
echo "MySQL started"

# Check if the media volume is empty, and if so, copy initial media files
if [ -z "$(ls -A /app/media)" ]; then
    echo "Initializing media directory..."
    cp -r /init-media/* /app/media/
fi

python manage.py migrate --no-input

gunicorn app.asgi:application \
    --bind 0.0.0.0:8000 \
    --workers 8 \
    --worker-class uvicorn.workers.UvicornWorker

exec "$@"

In this example, make sure your Dockerfile copies the initial media files into /init-media:

DOCKERFILECopied
COPY ./media /init-media

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2. Run a One-Time Data Seeder Container

Another approach is to create a dedicated service that runs once before the application starts to populate the volume.

Steps:

• Create a small Docker image (e.g., based on Alpine) that contains a script copying the contents from a source directory (which is in the image) into the media volume.
• Add this service to your Compose file and set it up to run before your main services (using dependencies or the “init” pattern).

Example addition to docker-compose.yaml:

YAMLCopied
services:
  media_seeder:
    image: alpine
    volumes:
      - media:/app/media
      - ./media:/init-media:ro
    entrypoint: ["sh", "-c", "if [ -z \"$(ls -A /app/media)\" ]; then cp -r /init-media/* /app/media/; fi"]

Then, make your backend service depend on this seeder:

YAMLCopied
services:
  backend:
    build: ./
    depends_on:
      - db
      - media_seeder
    volumes:
      - static:/app/static
      - media:/app/media
    ...

In this approach, the media_seeder container checks the media volume and copies over the files only if the volume is empty.

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Summary

There’s no built-in Docker Compose directive to automatically seed a named volume from the image’s filesystem, so you must implement this behavior (typically via an entrypoint script or a one-time data seeder container) to ensure your media volume is populated during the first run.

Choose the method that best fits your deployment and workflow.