Symfony 4.4 ignores env variables

I've just migrated my project from 3.4 to 4.4 and having a trouble to configure it correctly. My project is running inside a Docker container. I provide an env variable DATABASE_URL with a production url inside container:

env | grep DATABASE_URL - returns a correct value when I run this command inside the container.

Meanwhile my .env file contains:

...DATABASE_URL=dummy-url-for-local-development

But when I run composer dump-env it takes dummy url instead of the one from the docker container, yet the documentation clearly states that

Any existing env vars are never overwritten by the values defined in .env, so you can combine both.

What am I doing wrong?

P.S. I expect that any system env var would be prior to what I have in .env file.