📝 (Docker) Use same Dockerfile in lectures 2 and 3

Previously the Lecture 2 Dockerfile shown was more advanced and more "correct", however, we then use a different Dockerfile in practice in Lecture 3.

I think it's better to use the same, slightly less good Dockerfile, throughout the section, and then introduce the "better" Dockerfile when we deploy the app and use `gunicorn`.

Author: jslvtr

docs/docs/04_docker_intro/02_what_is_docker_image/README.md

@@ -8,11 +8,11 @@ Creating a Docker image is straightforward (when you know how!). I'll guide you
 
 ```dockerfile
 FROM python:3.10
+EXPOSE 5000
 WORKDIR /app
-COPY ./requirements.txt requirements.txt
-RUN pip install --no-cache-dir --upgrade -r requirements.txt
+RUN pip install flask
 COPY . .
-CMD ["gunicorn", "--bind", "0.0.0.0:80", "main:app"]
+CMD ["flask", "run", "--host", "0.0.0.0"]
 ```
 
 This is a `Dockerfile`, a definition of how to create a Docker image. Once you have this file, you can ask Docker to create the Docker image. Then, after creating the Docker image, you can ask Docker to run it as a container.