What is the hyphen after the ENV variable DOCKER_REGISTRY for in a docker-compose file

Question

In my docker-compose files, my image tag is like so

image: ${DOCKER_REGISTRY-}webapi

However the ENV variable is just DOCKER_REGISTRY, so what's the hyphen for?

Also, now I'm wanting to push these images to ECR, when I set the DOCKER_REGISTRY to my ECR instance, there is no /, between the DOCKER_REGISTRY and webapi part, so I guess I need to change it to look like image: ${DOCKER_REGISTRY-}/webapi? But maybe only for my non-dev docker-compose files?

user_1811501 · Answer

The - is part of the syntax for variable substitution, which is based on the standard described here: https://pubs.opengroup.org/onlinepubs/009604599/utilities/xcu_chap02.html#tag_02_06_02

${SOME_VAR-}

In this case, no default value is set, so it's redundant.

user_6791699 · Answer

Don't add a /. it's not a path, it's the name of the image.

What is the hyphen after the ENV variable DOCKER_REGISTRY for in a docker-compose file

2 Answers