How do i use secrets in Docker build?

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In my flask app I have

# python
password = os.environ.get('PASSWORD_MONGODB') or os.getenv("PASSWORD_MONGODB") 
username =  os.environ.get('USERNAME_MONGODB') or os.getenv("USERNAME_MONGODB")

And I need to use dockerfile, to build image. But it needs those two variables.
I should use secrets, but I’m confused by docs, what I should type, and how to simply use it as variables in code as well.

I’ve tried:

docker build backend/ --file backend/Dockerfile --tag my_backend_image:latest --secret id=USERNAME_MONGODB USERNAME_MONGODB="" --secret id=USERNAME_MONGODB USERNAME_MONGODB="usr"

but gives:
unknown flag: --secret

And for normal variables only this works:

ENV USERNAME_MONGODB "pass"

but i can’t put those in dockerfile, it needs to be passed as args, which doesn’t work (normal variables either):

Dockerfile:

ARG USERNAME_MONGODB
ARG PASSWORD_MONGODB

ENV MONGODB_USERNAME=${USERNAME_MONGODB}
ENV MONGODB_PASSWORD=${PASSWORD_MONGODB}

command:

docker run -d \
    -p 8080:5000 \
    --build-arg MONGODB_USERNAME=usr \
    --build-arg MONGODB_PASSWORD=pass \
    --name animanga_backend \
    animanga_backend:2.0

I get “None” for both variables, so not passed.

This works, but I use Render platform, so they just execute docker image
docker run -d -p 8080:5000 -e USERNAME_MONGODB='user' -e PASSWORD_MONGODB='pass' --name animanga_backend animanga_backend:2.0