Best practice for handling service configuration in docker

Question

I want to deploy a docker application in a production environment (single host) using a docker-compose file provided by the application creator. The docker based solution is being used as a drop-in replacement for a monolithic binary installer.

The application ships with a default configuration but with an expectation that the administrator will want to apply moderate configuration changes.

There appears to be a few ways to apply custom configuration to the services that are defined in the docker-compose.yml file however I am not sure which is considered best practice. The two I am considering between at the moment are:

• Bake the configuration into a new image. Here, I would add a build step for each service defined in the docker-compose file and create a minimal Dockerfile which uses COPY to replace the existing configuration files in the image with my custom config files. Using sed and echo in CMD statements could also be used to change configuration inline without replacing the files wholesale.

• Use a bind mount with configuration stored on the host. In this case, I would store all custom configuration files in a directory on the host machine and define bind mounts in the volumes parameter for each service in the docker-compose file.

The first option seems the cleanest to me as the application is completely self-contained, however I would need to rebuild the image if I wanted to make any further configuration changes. The second option seems the easiest as I can make configuration changes on the fly (restarting services as required in the container).

Is there a recommended method for injecting custom configuration into Docker services?

Answer 1

Given your context, I think using a bind mount would be better.

A Docker image is supposed to be reusable in different context, and building an entire image solely for a specific configuration (i.e. environment) would defeat that purpose:

• instead of the generic configuration provided by the base image, you create an environment-specific image
• everytime you need to change the configuration you'll need to rebuild the entire image, whereas with a bind mount a simple restart or re-read of the configuration file by application will be sufficient

• Docker documentation recommend that:

  Dockerfile best practice

  You are strongly encouraged to use VOLUME for any mutable and/or user-serviceable parts of your image.

  Good use cases for bind mounts

  Sharing configuration files from the host machine to containers.