Basic string variable substitution in docker-compose files (3.8) - docker-compose

Question

Take my example of two services:
services:
nginx:
ports:
- 443:443
volumes:
- "CONFIG_DIRECTORY/nginx/nginx.conf:/etc/nginx/nginx.conf"
- "CONFIG_DIRECTORY/certs:/etc/ssl/certs"
web:
command: ["node", "index.js"]
volumes:
- "CONFIG_DIRECTORY/certs:/var/client/config/ssl/certs"
- "CONFIG_DIRECTORY/process:/var/client/process"
I'd really like to be able to substitute a string such as /home/garnettm/development/config directly into the indicated CONFIG_DIRECTORY locations in the above strings.
Is there any way to do this other than the many currently available environment variable substitution process options?
A .env file for example would allow you to do this using an already defined variable and the $VARIABLE syntax.

Answer 1

The only form of string interpolation Compose supports at all is environment variable substitution. This uses shell-style $VARIABLE syntax, with very limited options for if the variable isn't set, ${VARIABLE:-default value} or ${VARIABLE:?error message}.
There is no way to declare these variables in the Compose file itself. Compose does support putting variable values in a .env file, so they don't necessarily need to be set as actual environment variables. You do not need to mention this file in a Compose env_file: directive, Compose reads it on its own to set environment variables to do these substitutions.
# .env
CONFIG_DIRECTORY=/home/garnettm/development/config
# docker-compose.yml
volumes:
- "$CONFIG_DIRECTORY/nginx/nginx.conf:/etc/nginx/nginx.conf"
For this specific setting, on the left-hand side of volumes: and other places that refer to host directories, relative paths are interpreted relative to the location of the Compose file (the first one, if you have multiple docker-compose -f options). A very common setup is to put all of the state and source code in the same directory tree, and to put the docker-compose.yml file at the root of that tree. In that case the relative directory . will almost always be right
volumes:
- "./nginx/nginx.conf:/etc/nginx/nginx.conf"
YAML anchors are occasionally used to provide common blocks of settings, but you can't do string interpolation using anchors, only supply an entire YAML scalar node. They won't be helpful here.