FastApi container not running in a Container Instance

I have created a docker image of a simple FastApi app, based in the example by Microsoft (using gunicorn as server), and also other FastApi made by me using only uvicorn as server.

I got this message: "one or more of the containers in XXXXX are in a Waiting state and may not be running"

I have several different configurations but always same result.

If somebody can help me it would be very appreciated.

The main.py is very simple.

from fastapi import FastAPI
import uvicorn

app = FastAPI()


@app.get("/")
async def root():
    return {"msg": "Hola FastAPI!!"}


@app.get("/url")
async def url():
    return {"url": "http://midomain.es"}


if __name__ == "__main__":
    uvicorn.run("main:app", host="0.0.0.0", port=8000)

And this is the Dockerfile:

# syntax=docker/dockerfile:1

FROM python:3.11

WORKDIR /code

COPY requirements.txt .

RUN pip install --no-cache-dir --upgrade -r requirements.txt

COPY . .

EXPOSE 3100

CMD ["gunicorn" ,"main:app"]

Finally this is the gunicorn.conf.py:

# Gunicorn configuration file
import multiprocessing

max_requests = 1000
max_requests_jitter = 50

log_file = "-"

bind = "0.0.0.0:3100"

worker_class = "uvicorn.workers.UvicornWorker"
workers = (multiprocessing.cpu_count() * 2) + 1