TIL: How to Use Default Values in docker-compose.yml
Sometimes we want to use env variables in our docker-compose files like so:
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:
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.