Docker File and Python - python-3.x

Question

Apologies I am very new to Docker. I have the following Docker file which contains the following commands (see below). I am not sure I understand all commands and I would appreciate some explanation. I commented all the lines I understood but put a question mark in others. Please see below
#That this line means that python will be our base. Can some comment here please and explain this line more?
FROM python:3.9 as base
#create a working directory in the virtual machine (VM)
WORKDIR /code
# copy all the python requirements stored in requirements.txt into the new directoy (in the VM)
COPY ./requirements.txt /code/requirements.txt
# activate the package manager pip. But why use no-cache-dir?
RUN pip install --no-cache-dir --upgrade -r /code/requirements.txt
# copy all files to the new directory (in the VM)
COPY ./ /code/
# I don't understand the line below. Please explain? why uvicorn? app.main:app is the
#location of the fastapi
CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "180"]
Thank you

Answer 1

A Docker file states all steps that Docker will execute when creating your image. From that image, a container can be created.
#That this line means that python will be our base. Can some comment here please and explain this line more?
FROM python:3.9 as base
This is very basic docker stuff, follow a (beginners) tutorial and you will learn a lot more than just someone spoon-feeding little bits of knowledge.
#create a working directory in the virtual machine (VM)
WORKDIR /code
You are creating a container image, not a VM. That is a similar but very different concept and should not be mixed.
# copy all the python requirements stored in requirements.txt into the new directoy (in the VM)
COPY ./requirements.txt /code/requirements.txt
This copies all files to the image.
# activate the package manager pip. But why use no-cache-dir?
RUN pip install --no-cache-dir --upgrade -r /code/requirements.txt
RUN is an image build step, and the outcome will be committed to the Docker image. So, in this step, you are telling docker that you want an image that has everything installed as outlined in requirements.txt with pip. No cache, by default PIP saves the whl's of the packages you are installing, but that only would increase the image and are no longer required. So, no cache.
# copy all files to the new directory (in the VM)
COPY ./ /code/
Again, not VM but image, an image that will later be used to create a container.
# I don't understand the line below. Please explain? why uvicorn? app.main:app is the
#location of the fastapi
CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "180"]
Because you are trying to run a FastAPI project, and FastAPI is just the app; you need a server to actually be able to fire request at FastAPI. This is explained on the very first page of the FastAPI documentation actually.

Answer 2

"app.main:app" express your project has such python file:
<Project Root Dir>
app - folder
main.py -- python file
In the main.py, you init a FastAPI instance and named app, like this:
# main.py
....
app = FastAPI()
...
unicorn use above rules to get the FastAPI instance app, then load it.