Introduction

There is a nice post on the many available ways of passing in application configuration into a docker container. Here is another one, which is similar to the ones listed in the post but still possibly stands out as a different technique.

App config via entrypoint

The idea is that we pass in environment variables to a script that runs as the container entrypoint. This entrypoint script generates a settings JSON file for the application to use as its configuration.

Docker file with nginx base (can be any other base, but this example is nginx specific)

FROM nginx:1.23.4-alpine-slim
COPY ./ /src/app/
WORKDIR /src/app
RUN chmod +x settings.sh
ENTRYPOINT ["sh","/src/app/settings.sh"]

The settings.sh

#!/bin/sh
# expecting these variables are set via environment
echo "{ \"SETTING1\": \"$SETTING1\", \"SETTING2\": \"$SETTING2\" }" > settings.json

# display file to screen for verification
cat settings.json

echo "running nginx command"
set -e
exec nginx -g "daemon off;"
echo "finished running nginx command"

The exec nginx -g "daemon off;" is the crucial bit. As we are providing an entrypoint, the entrypoint of the base image won’t run. So we need to do what we expect out of the base.

Docker build, say we call this image dockerpoc

docker build -t dockerpoc:1.0 --no-cache .

Finally, we pass in the required values via environment variables during the docker run.

docker run -e SETTING1='setting1-value' -e SETTING2='setting2-value' -it dockerpoc:1.0 

This creates the configuration for the application in a JSON format at the docker container runtime.

Conclusion

Hope this was useful and saves you some time. Please do share your learnings. If you have any thoughts or comments, please do get in touch with me on Twitter @rubberduckdev. Or use the Disqus plugin below.

Credits

Banner by Rubaitul Azad