docker-compose, multiple yml files and environment variables

Did you know that docker-compose can merge multiple yml files into one? it seems like a very cool way to override configs and have something like a linear composability.

Just for fun, try

And do a few tests using

• docker-compose -f docker-compose.yml config
• docker-compose -f docker-compose.yml -f docker-compose-over.yml config
• FOO=1 docker-compose -f docker-compose.yml config
• FOO=1 docker-compose -f docker-compose.yml -f docker-compose-over.yml config
• docker-compose -f docker-compose.yml up
• docker-compose -f docker-compose.yml -f docker-compose-over.yml up
• FOO=1 docker-compose -f docker-compose.yml up
• FOO=1 docker-compose -f docker-compose.yml -f docker-compose-over.yml up

In addition to this, you can use the .env file to add defaults to the variables, and you can take a look at a special .override.yml suffix in https://docs.docker.com/compose/extends/. Plenty of ways to configure, reconfigure, and override variables. The difficult part is to make it understandable at all for the next guy touching your setups.

Also, in your app, you can have defaults for the env variables (docker will set them to empty strings if you don't set a value for them).

Also, remember that "docker-compose run" and "docker-compose exec" accept -e to set environment variables. I still haven't tried all the possibilities with those to see exactly what do they overwrite and what is fixed.