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How to access host port after running docker compose up

I am dockerizing a React-Flask web application with separate containers for the frontend and the backend Flask API. Up to now I have only run this on my localhost using the default Flask development server. I then installed Gunicorn to prep the application for deployment with Docker later, and that also ran smoothly on my localhost.
After I ran docker compose up the two images built successfully, are attached to the same network, and I got this in the logs:
Logs For backend:
Starting gunicorn 20.1.0
Listening at: http://0.0.0.0:5000 (1)
Using worker: gthread
Logs For frontend:
react-flask-app#0.1.0 start
react-scripts start
Project is running at http://172.21.0.2/
Starting the development server...
But when I try to access the site at http://172.21.0.2/, localhost:5000 or localhost:3000 it is not accessible. Do I maybe need to add the name of the frontend or backend service?
In Docker Desktop it's showing that the frontend is running at port 5000 but there is no port listed for the backend, it just says it's running.
This is what my files and setup look like:
I added a gunicorn_config.py file as I read it is a good practice, rather than adding all of the arguments to the CMD in the Dockerfile:
bind = "0.0.0.0:5000"
workers = 4
threads = 4
timeout = 120
Then in my Flask backend Dockerfile I have the following CMD for Gunicorn:
FROM python:3.8-alpine
EXPOSE 5000
WORKDIR /app
COPY requirements.txt requirements.txt
ADD requirements.txt /app
RUN pip install --upgrade pip
ADD . /app
COPY . .
RUN apk add build-base
RUN apk add libffi-dev
RUN pip install -r requirements.txt
CMD ["gunicorn", "--config", "gunicorn_config.py", "main:app"]
Here I do "main:app" where my Flask app file is called main.pyand then app is my Flask app object.
I'm generally confused about ports and how this will interact with Gunicorn and in general. I specified port 5000 in the EXPOSE of both of my Dockerfiles.
This is my frontend Dockerfile:
WORKDIR /app
COPY . /app
RUN npm install --legacy-peer-deps
COPY package*.json ./
EXPOSE 3000
ENTRYPOINT [ "npm" ]
CMD ["start"]
And used 5000 in the bind value of my Gunicorn config file. Also, I previously added port 5000 as a proxy in package.json.
I will initially want to run the application using Docker on my localhost but will deploy it to a public host service like Digital Ocean later.
This is my Docker compose file:
services:
middleware:
build: .
ports:
- "5000:5000"
frontend:
build:
context: ./react-flask-app
dockerfile: Dockerfile
ports:
- "3000:3000"
The other thing to mention is that I also created a wsgi.py file and I was wondering do I need to add this to the Gunicorn CMD in my Dockerfile:
from main import app
if __name__ == "__main__":
app.run()

Docker connection refused on all ports except 5000

I have just started with Docker. I have installed Docker Toolbox for Windows. I was trying out a sample Flask app to understand how things work. But I am stuck!. I am trying to access my app like this http://docker-machine-ip : port number but every time I do, I get '{docker-machine ip} refused to connect.'
I get no exceptions during the building and deploying stages. I also did docker ps to see that container is running. I even tried to access it via Kitematic but still no luck. Below are details related to the app
app.py
from flask import Flask
app = Flask(__name__)
#app.route("/")
def hello():
return "Flask inside Docker shakel!!"
if __name__ == "__main__":
app.run(debug=True,host='0.0.0.0')
requirements.txt
flask
Dockerfile
FROM python:2.7
MAINTAINER Shekhar Gulati "shekhargulati84#gmail.com"
COPY . /app
WORKDIR /app
RUN pip install -r requirements.txt
ENTRYPOINT ["python"]
CMD ["app.py"]
The docker commands I used for building and running are:
docker-machine ip default //To get docker machine ip
docker build -t flask-app .
docker run -d -p 6000:6000 flask-app
I have Windows 7 64 bit. Please let me know if any more info is required.
P.S. However I noticed that if I map my container to 5000 port it will run fine but on any other port I get connection refused
I don't know what am I missing here. This is my first attempt at Docker and I have googled for 4 hrs to find a solution but nothing is working. So it might be a very dumb mistake I am doing somewhere :).anyhow any help is very much appreciated. Thanks in advance.

That's because you didn't set the port for your flask application, which is port 5000 by default.
From flask documentation:
port – the port of the webserver. Defaults to 5000 or the port defined in the SERVER_NAME config variable if present.

Connection Refused Docker Run

I'm getting a connection refused after building my Docker image and running docker run -t imageName
Inside the container my python script is making web requests (external API call) and then communicating over localhost:5000 to a logstash socket.
My dockerfile is really simple:
FROM ubuntu:14.04
RUN apt-get update -y
RUN apt-get install -y nginx git python-setuptools python-dev
RUN easy_install pip
#Install app dependencies
RUN pip install requests configparser
EXPOSE 80
EXPOSE 5000
#Add project directory
ADD . /usr/local/scripts/
#Set default working directory
WORKDIR /usr/local/scripts
CMD ["python", "logs.py"]
However, I get a [ERROR] Connection refused message when I try to run this. It's not immediately obvious to me what I'm doing wrong here - I believe I'm opening 80 and 5000 to the outside world? Is this incorrect? Thanks.

Regarding EXPOSE:
Each container you run has its own network interface. Doing EXPOSE 5000 tell docker to link a port 5000 from container-network-interface to a random port in your host machine (see it with docker ps), as long as you tell docker to do it when you docker run with -P.
Regarding logstash.
If your logstash is installed in your host, or in another container, it means that logstash is not in the "localhost" of the container (remember that each container has its own network interface, each one has its own localhost). So you need to point to logstash properly.
How?
Method 1:
Don't give container its own iterface, so it has the same localhost as your machine:
docker run --net=host ...
Method 2:
If you are using docker-compose, use the docker network linking. i.e:
services:
py_app:
...
links:
- logstash
logstash:
image: .../logstash..
So point as this: logstash:5000 (docker will resolve that name to the internal IP corresponding to logstash)
Method 3:
If logstash listen in your localhost:5000 (from your host), you can point to it as this: 172.17.0.1:5000 from inside your container (the 172.17.0.1 is the host fixed IP, but this option is less elegant, arguably)

Cannot hit docker container running locally

Here's my Dockerfile:
FROM alpine:3.5
RUN apk add --no-cache python3 && \
python3 -m ensurepip && \
rm -r /usr/lib/python*/ensurepip && \
pip3 install --upgrade pip setuptools && \
rm -r /root/.cache
# Copy files
COPY ./requirements.txt /app/requirements.txt
COPY ./main.py /app/main.py
# Install deps
RUN pip install --upgrade pip
RUN pip install --requirement /app/requirements.txt
# Set the default directory where CMD will execute
WORKDIR /app
EXPOSE 5000
CMD [ "python3", "-u", "./main.py" ]
and my main.py file:
from flask im
port Flask
app = Flask(__name__)
#app.route("/")
def hello():
return "Hello World!"
if __name__ == "__main__":
app.run()
After building the tag and running it via docker run <tag>, get logs saying the flask server was successfully started:
$ docker run test:latest
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
But I cannot hit the server in my browser.
I thought the problem may be in the port mapping, so I ran the container like this:
$ docker run -p 5000:5000 test:latest
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
Still no dice. I am able to run the server outside of the container, simply by running python3 main.py. what am I doing wrong?

The script is listening to 127.0.0.1 in the container, making it inaccessible from the host. It must listen to 0.0.0.0.

The 127.0.0.1 address that you see is the localhost of the container networking, that is not the same as the main host. When exposing ports, docker maps a port binding in container internal IP (172.17....) to a port in the hosts interfaces.
So you must tell your flask to listen to all of its interfaces:
app.run(host='0.0.0.0')

127.0.0.1 is the loopback address (also known as localhost).
0.0.0.0 is a non-routable meta-address used to designate an invalid, unknown, or non-applicable target (a ‘no particular address’ place holder). For things like address binding of network connections, the result can be to assign an appropriate interface address to the connection.
In Docker, the loopback address (127.0.0.1) of a container is not the same as the host. Use 0.0.0.0 as the host instead.
app.run(host='0.0.0.0')

Docker container in-app port forwarding

I actually trying to make port forwarding for Docker which make request to mysql database on a remote server.
My goal is to create a copy of an app production in a container which can has access to an existent MySQL server based on a remote one.
I create a Dockerfile with all the requirements embedded, as follow :
FROM python:2.7-alpine
mkdir /working_env
RUN apk add --update mysql\
mysql-client\
python-dev\
build-base \
&& rm -rf /var/cache/apk/*
# Requirements here
RUN pip install -r requirements.txt
WORKSPACE working_env
COPY init.sh .
# BIND TO 8053
RUN init.sh start
and run this container with a volume mapped to the local project
docker run --name --rm project -v $(pwd):/working_env -d -p 8053:8053 project_docker
the call is made in my code as follows :
conn = MySQLdb.connect(host=10.11.12.13, port=3000, passwd=foo, user=admin, db=default)
But I want to be able when the call is made from inside my container, that it's been replaced from 10.11.12.13:3000 to 10.11.12.13:3001.
I've found topics about iptables and tries to create rules, but all my tries have failed in order to forward this port.
sudo iptables -t nat -A PREROUTING -s 127.0.0.1 -p tcp --dport 3000 -j REDIRECT --to 3001
Does it even possible to made this kind of modification or is it a another way to alter the port from inside the docker?