Pass codecov variables to docker build

I would like to pass the coverage report to codecov during docker build phase. Like this:

ci_env=`bash <(curl -s https://codecov.io/env)`

docker build . --build-arg ci_env=$ci_env

This is currently not possible since ci_env is a list of environment variables.

In the documentation they describe how to achieve it with docker run but not with docker build.

Does anyone know how to do this?