Why does the error "python: can't open file '/app/${FILENAME}': [Errno 2] No such file or directory" occur?

Question

Docker

I have an image that runs python scripts, which takes an environment variable FILENAMEto know which script to run.

Dockerfile :

FROM python
ARG FILENAME_ARG
ENV FILENAME = $FILENAME_ARG
WORKDIR /app
COPY . .
CMD ["python", "${FILENAME}"]

In the console I enter the command:

docker run --env FILENAME=hello_world.py *Image ID*

An error occurs:

python: can't open file '/app/${FILENAME}': [Errno 2] No such file or directory

I assumed that by creating the container in this way, the variable FILENAME
would take on the value hello_world.py. But, apparently, something is going wrong.

Where is the mistake and how to fix it?