How to pass a YML config file in Docker Image during run - docker

Question

I have built a docker image. It has a YML config file which is a large configuration something like this
contrydata:
continent: europe
country: luxembourg
county: luxembourg
town: luxembourg
...
I am using this configuration file during the run of docker. I want to use this as a default configuration. If any user wants to change the configuration how does this config file passes to the docker image during the run.
I have seen a few docker run command like this which uses an external config file
docker run --rm -it -v $(pwd):/data -p 80:80 dockerhub/mydockerimage:v3 --config config.yml
Where users can configure this file and can pass it from an external source.
In a nutshell, if users pass a config file then the default config file will be replaced with the provided config.yml file otherwise the default config.yml file will be used.

Answer 1

The way this is usually done, as far as I can tell, is to map the config file to a specific location in the container. Nginx does this and has a volume mapping on the docker run command like this -v /host/path/nginx.conf:/etc/nginx/nginx.conf:ro. The Nginx container then always looks for the config file in the same location - /etc/nginx/nginx.conf.
I've never seen it passed as a parameter as you show. Those parameters will be passed to the program defined as the entrypoint in the container. That program doesn't have access to the host file system, so unless the config file has been mapped to the container using a volume, the program won't have access to the file.
Edit:
Just for the fun of it, I tried making this Dockerfile that builds a small C program that prints out any arguments passed to it. I also created a shell script that passes any arguments on to pgm. This is pretty much how Tileserver does it.
FROM ubuntu
RUN apt-get update && apt-get install -y gcc
WORKDIR /app
RUN echo '#include <stdio.h> \n\
int main(int argc, char **argv) { \n\
for (int i=0; i<argc; i++) { \n\
printf("%s\\n", argv[i]); \n\
} \n\
}' > pgm.c
RUN gcc -o pgm pgm.c
RUN echo '#!/bin/bash\n\
./pgm "$#"' > docker-entrypoint.sh
RUN chmod 775 docker-entrypoint.sh
ENTRYPOINT ["./docker-entrypoint.sh"]
If you build and run it using the commands
docker build -t test .
docker run --rm test --config config.file
it prints out
./pgm
--config
config.file
If you wanted to do it like Tileserver, the C program would then need to open config.file and use the contents. But of course it would only be able to get at it, if the file is mapped using a volume.