TIL: How to Use Default Values in docker-compose.yml

TIL: How to Use Default Values in docker-compose.yml

Sometimes we want to use env variables in our docker-compose files like so:

services:
    client:
        image: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/nginx
        ports:
            - 8000:80

Here we are going to use the GitLab CI dependency proxy to pull our Nginx image, so we can speed up our pipelines but also avoid being rate limited by docker hub. However, when running this locally, we will need to make sure the CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX env variable is set. Which just adds a bit more work, instead we can leverage some of the special syntax docker-compose provides ¹, which I think it inherits from bash.

Where we can use interpolation to set default values if the env variable is not set:

services:
    client:
        image: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX:-docker.io}/nginx
        ports:
            - 8000:80

In this case, :- will use docker.io if the env variable is not set or is empty. There are several other variations you can find in the footnote below.