Read JSON-file into environment variable with Docker Compose - json

Question

I've dockerized a Meteor-app with Meteord, and that works fine, my problem is that I want to pass some settings to the app.
Meteord does not start the app with a settings-file as one would usually do to give settings to an app (meteor --settings file.json). This is also possible to do with an environement variable called METEOR_SETTINGS.
As I want the webapp to run with other services, I'm using Docker Compose.
I have my settings.json-file that I want to be read in as a environment variable, so something like:
environment:
- METEOR_SETTINGS=$cat(settings.json)
This doesn't work though.
How can I make Docker compose dynamically create this environment variable based on a JSON-file?

Answer 1

An easy way to do this is to load the JSON file in to a local env var, then use that in your yaml file.
In docker-compose.yml
environment:
METEOR_SETTINGS: ${METEOR_SETTINGS}
Load the settings file before invoking docker-compose:
❯ METEOR_SETTINGS=$(cat settings.json) docker-compose up

Answer 2

Not possible without some trickery, depending on the amount of tweakable variables in settings.json:
If it's a lot of settings it's fairly easy to template the docker-compose.yml with a simple shell script that replaces a token in the template with the contents of settings.json, much like in your example. You also want to wrap the docker-compose call in that case. Simplified example:
docker-compose.yml.template:
environment:
- METEOR_SETTINGS=##_METEOR_SETTINGS_##
dc.sh:
#!/bin/sh
# replace ##_METEOR_SETTINGS_## with contents of settings.json and output to docker-compose.yml
sed -e 's|##_METEOR_SETTINGS_##|'"$(cat ./settings.json)"'|' \
"./docker-compose.yml.template" > "./docker-compose.yml"
# wrap docker-compose, passing all arguments
docker-compose "$#"
Put the 2 files into your project root, then chmod +x dc.sh to make the wrapper executable and call ./dc.sh -h.
If it's only a few settings you could handle the templating inside the container when its starting. Simply replace tokens placed in a prepared settings.json with ENV values passed to docker before starting Meteor. This allows you to just use the docker-compose ENV features to configure Meteor.