run command after mounting volumes

Im trying to build a container image that will download a git repo and move it to a specific location in a container. I am successfully able to build the image and downloading the repo but the coping of the file is failing and i am not getting any error. The location the file is to be copied to is a mounted volume from the host so what i assume what is happening is the directory is not there when it is trying to copy it so its failing. I am using Dockerfile to create the image but the files never seem to be copied but the repo is downloaded.

I have attempt to use the CMD to copy the script but same result and also tried using a script with the copy command that starts with a sleep timer of 10 seconds but the files are never copied.

The container does have read/write to the mount as the app is updating the config in the mount.

Here is the contents of my Dockerfile

FROM ubuntu:22.04
RUN apt-get -y install git && git clone "https://github.com/user/repo.git" 
RUN cp /repo/file.py /config/webapp/cleanup_script/
RUN pip3 install x, y, z 
# keeping the container running.
CMD sleep infinity

Any idea what i am doing wrong here?

docker-compose file.

services:
  webapp:
    image: 0594dd495719
    domainname: webapp.serbio
    container_name: webapp
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=Europe/London
      - UMASK_SET=000
      - WEBUI_PORT=8080
    volumes:
      - /configs/webapp:/config/
      - /mnt:/mnt/
    ports:
      - 6881:6881
      - 6881:6881/udp
      - 8080:8080
      - 51111:51111
      - 51111:51111/udp
    networks:
       vpcbr:
        ipv4_address: 10.1.1.10
    restart: unless-stopped

This image will be run in docker and in kubernetes.

Solution

The Dockerfile runs before any volumes are created or mounted. In the Dockerfile you can't change volume content, this needs to happen when the container starts.

The usual pattern I use here is an entrypoint wrapper script. You can write a shell script that does the startup-time setup, and then switches to the main container command. This runs at container startup, so it's after the volume is created and mounted.

#!/bin/sh
if [ ! -d /config/webapp ]; then
  mkdir /config/webapp || exit 1
fi
if [ ! -d /config/webapp/cleanup_script ]; then
  mkdir /config/webapp/cleanup_script || exit 1
fi
if [ ! -f /config/webapp/cleanup_script/file.py ]; then
  cp /repo/file.py /config/webapp/cleanup_script || exit 1
fi

exec "$@"

In your Dockerfile, make this script be the ENTRYPOINT. The ENTRYPOINT line must use the JSON-array syntax. Leave your CMD unchanged.

ENTRYPOINT ["./entrypoint.sh"]
CMD ["./webapp"]

This will work so long as the config directory is writable. You mention Kubernetes; an emptyDir volume will work (but won't survive restarts), or you can create a PersistentVolumeClaim via a StatefulSet, but you can't mount a ConfigMap here because it won't be writable.