Docker-compose volumes working with Dockerfile : Device or resource busy - docker

Question

I get an issue working with docker-compose service while using Dockerfile build.
Indeed, I provide a .env file into my app/ folder. I want the TAG value of the .env file to be propagate/render into the config.ini file. I tried to achieve using entrypoint.sh (which is launch just after the volumes) but it failed.
There is my docker-compose.yml file
# file docker-compose.yml
version: "3.4"
app-1:
build:
context: ..
dockerfile: deploy/Dockerfile
image: my_image:${TAG}
environment:
- TAG=${TAG}
volumes:
- ../config.ini:/app/config.ini
And then my Dockerfile:
# file Dockerfile
FROM python:3.9
RUN apt-get update -y
RUN apt-get install -y python-pip
COPY ./app /app
WORKDIR /app
RUN pip install -r requirements.txt
RUN chmod 755 entrypoint.sh
ENTRYPOINT ["/app/entrypoint.sh"]
CMD ["python", "hello_world.py"]
In my case, I mount a config.ini file with the configuration like :
# file config.ini
[APP_INFO]
name = HELLO_WORLD
version = {TAG}
And finally, in my app folder, I have a .env file where you can found the version of the app, which is evoluing through time.
# file .env
TAG=1.0.0
Finally
#!/bin/bash
echo "TAG:${TAG}"
awk '{sub("{TAG}","${TAG}")}1' /app/config.ini > /app/final_config.ini
mv /app/final_config.ini /app/config.ini
exec "$#" # Need to execute RUN CMD function
I want my entrypoint.sh (which is called before the last DOCKERFILE line and after the docker-compose volumes. With the entrypoint.sh, I want overwritte my mounted file by a new one, cerated using awk.
Unfortunatly, I recover the tag and I can create a final_config.ini file, but I'm not able to overwrite config.ini with it.
I get this error :
mv: cannot move '/app/final_config.ini' to '/app/config.ini': Device or resource busy
How can I overwritting config.ini without getting error? Is there an more simple solution?

Answer 1

Because /app/config.ini is a mountpoint, you can't replace it. You should be able to rewrite it, like this...
cat /app/final_config.ini > /app/config.ini
...but that would, of course, modify the original file on your host. For what you're doing, a better solution is probably to mount the template configuration in an alternate location, and then generate /app/config.ini. E.g, mount it on /app/template_config.ini:
volumes:
- ../config.ini:/app/template_config.ini
And then modify your script to output to the final location:
#!/bin/bash
echo "TAG:${TAG}"
awk '{sub("{TAG}","${TAG}")}1' /app/template_config.ini > /app/config.ini
exec "$#" # Need to execute RUN CMD function