mkdir permission denied inside docker container

Question

I'm trying to create POST action to upload images in Django. Using python:3.9-alpine3.13 docker image.

Dockerfile contains those lines:

RUN \
adduser \
        --disabled-password \
        --no-create-home \
        djuser && \
    mkdir -p /vol/web/media && \
    mkdir -p /vol/web/static && \
    chown -R djuser:djuser /vol && \
    chmod -R 755 /vol

USER djuser

After this I did docker-compose build and tried this command:

docker-compose run --rm app sh -c "whoami && cd ../vol/web && ls -l && mkdir ./media/uploads"

And got this

djuser
total 8
drwxr-xr-x    2 755      djuser        4096 Dec  5 11:07 media
drwxr-xr-x    2 755      djuser        4096 Dec  5 11:07 static
mkdir: can't create directory './media/uploads': Permission denied

So as I see: I tried mkdir from django-user that have 755 permission on directory where I want to create a new directory. Any thoughts why this doesn't work?

Creating directories using sudo and after that chmod them is not a good idea here (i think) because script must create directories. And using sudo or changing to the root everytime...