docker exec sees changes in local files

Question

I am playing with django project in docker and reality of the situation contradicts my limited understanding:
after I change file tests.py and run
docker exec {my_container} python manage.py test
it knows these local changes that I've made (for example I can write new test and it will run).
But I thought that docker containers are isolated and I need to recreate or at least copy updated file to one in order to update state of files.

I read docker exec doc but found no info about its ability to update files in container.
So, what do I need to know in order to understand what is happening?

---

EDIT

content of Dockerfile:

FROM python:3
ENV PYTHONUNBUFFERED 1
RUN mkdir /code
WORKDIR /code
ADD . /code/
RUN pip install -r requirements.txt

docker-compose.yml:

version: '3'
services:
  ankete_db:
    image: postgres
    ports: 
      - "54320:5432"
    container_name: ankete_db
  ankete:
    build: .
    container_name: ankete
    command: gunicorn -b 0.0.0.0:8000 ankete.wsgi
    volumes:
      - .:/code
    ports:
      - "8000:8000"
    depends_on:
      - ankete_db

Answer 1

volumes in your docker-compose.yml does this. Once you remove volumes your changes shuldn't be reflected in a container.

Answer 2

The exec command does not update files between host and container. What is causing the files to be synced between container and host is the volume declared inside the compose file.

The volume declaration mounts the directory . onto /code inside the container, such that any changes done to any file inside the host current directory are reflected in the /code directory inside the container and vice versa.