Description

I found an important difference in docker-compose behavior regarding default variables set in images, since v2.24.7.

Knowing that we have an image containing a variable "VAR" with a default value "default", the Dockerfile specifies:
ENV VAR "default"

If the environment section of the docker-compose manifest specifies a variable "VAR" (without value):

• Before v2.24.7: the variable is unset from the container (the "default" value is lost)
• Since v2.24.7: the variable with its default value is kept in the container

Regardless of the version, if the environment section of the docker-compose manifest specifies "VAR=" (with the equal sign and no value), the variable is set as empty in the container.

This difference in behavior is important because:

• If you deploy your images with docker-compose or another orchestrator you want your default variables being defined in images (not in .env or docker-compose manifest).
• If you want to override one of these variables just by setting it in .env or in the execution context (export VAR=...) you need to specify it in the environment section of the docker-compose manifest, if not the variable will not be passed to the container.

For me the behavior since v2.24.7 is the good one, but it doesn't appear in changelog, I don't even not know which issue or commit changed this, it's maybe just a side effect.

Can you confirm this behavior introduced by v2.24.7 is the expected one and can you add it to the official documentation ?
If a unit test could ensure this behavior will not be lost it would be great.