Why Docker improves development workflow - Why It Works

Introduction

Developers often face problems when their apps work on their computer but not on others or on servers. Docker solves this by packaging apps with everything they need to run, making the app behave the same everywhere.

When you want to share your app with teammates and ensure it runs the same on their computers.

When you need to run multiple apps on one machine without them interfering with each other.

When you want to test your app in an environment identical to the production server.

When you want to quickly start and stop your app without installing many dependencies.

When you want to deploy your app easily to cloud servers or other machines.

Config File - Dockerfile

Dockerfile

FROM python:3.11-slim
WORKDIR /app
COPY . /app
RUN pip install --no-cache-dir flask
CMD ["python", "app.py"]

This Dockerfile starts from a small Python image.

It sets the working folder inside the container to /app.

It copies your app files into the container.

It installs Flask, a Python web framework.

Finally, it runs your app using Python.

Commands

This command builds a Docker image named 'my-flask-app' from the Dockerfile in the current folder. It packages your app and its environment.

Terminal

docker build -t my-flask-app .

Expected Output

Sending build context to Docker daemon  4.096kB

Step 1/5 : FROM python:3.11-slim

 ---> 123abc456def

Step 2/5 : WORKDIR /app

 ---> Using cache

 ---> 789def012abc

Step 3/5 : COPY . /app

 ---> Using cache

 ---> 345ghi678jkl

Step 4/5 : RUN pip install --no-cache-dir flask

 ---> Running in abc123def456

Collecting flask

  Downloading Flask-2.3.2-py3-none-any.whl (96 kB)

Installing collected packages: flask

Successfully installed flask-2.3.2

Removing intermediate container abc123def456

 ---> 901mno234pqr

Step 5/5 : CMD ["python", "app.py"]

 ---> Running in stu567vwx890

Removing intermediate container stu567vwx890

 ---> 345yz123abc

Successfully built 345yz123abc

Successfully tagged my-flask-app:latest

→

-t - Names and tags the image for easy reference

This command runs your app inside a container and maps port 5000 of your computer to port 5000 inside the container so you can access the app in your browser.

Terminal

docker run -p 5000:5000 my-flask-app

Expected Output

* Serving Flask app 'app'

* Debug mode: off

WARNING: This is a development server. Do not use it in a production deployment.

* Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

→

-p - Maps container port to host port for access

This command lists running containers so you can check that your app container is running.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE          COMMAND                  CREATED          STATUS          PORTS                    NAMES

345yz123abc    my-flask-app   "python app.py"          10 seconds ago   Up 9 seconds    0.0.0.0:5000->5000/tcp   loving_morse

This command stops the running container named 'loving_morse' to free resources or update the app.

Terminal

docker stop loving_morse

Expected Output

loving_morse

Key Concept

Docker packages your app with everything it needs so it runs the same everywhere, avoiding 'it works on my machine' problems.

Common Mistakes

Not mapping ports with -p when running the container

Without port mapping, you cannot access the app from your computer's browser.

Always use -p host_port:container_port to expose the app port.

Forgetting to rebuild the image after changing app files

The container runs the old code because the image was not updated.

Run 'docker build' again to create a new image with changes.

Running containers without stopping old ones

Multiple containers may use the same ports causing conflicts.

Stop old containers with 'docker stop' before starting new ones.

Summary

Build a Docker image to package your app and its environment.

Run the image as a container with port mapping to access the app.

Use 'docker ps' to check running containers and 'docker stop' to stop them.