#100daysofSRE (Day 23): Modernize and Containerize your Applications or Microservices using Docker

Docker has transformed how applications are packaged, shipped, and run by introducing containers—lightweight, portable environments that encapsulate everything an application needs to run.

It also solves the major issue of running an application on different machines. Docker provides the same portability option so that you can run your application on Linux, Windows, or whatever platform you want to!

But how does Docker compare to traditional Virtual Machines (VMs), and why should SREs and DevOps engineers prefer it?

In this post, we’ll discuss why Docker is better than VMs, and then we’ll write a Dockerfile from scratch to containerize a simple application.

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🚀 Why Docker is Better Than Virtual Machines

Feature	Docker 🐳	Virtual Machines 💻
Startup Time	Seconds	Minutes
Performance	Lightweight, shares host OS	Heavy, each VM runs its own OS
Resource Usage	Low (efficient memory & CPU usage)	High (entire OS per VM)
Portability	Runs anywhere (on any OS, cloud, on-prem)	Tied to specific infrastructure
Scalability	Easily scales with orchestration (Kubernetes)	Scaling requires provisioning full VMs
Isolation	Process-level isolation	Full OS-level isolation

✅ Key Advantages of Docker:

• Fast Deployment: Containers spin up in seconds.
• Less Overhead: No need for an entire OS per application.
• Consistent Environments: Works the same on any system.
• Easy to Scale: Works seamlessly with Kubernetes and cloud providers.

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🛠 Writing Your First Dockerfile

A Dockerfile is a script containing a set of instructions to build a Docker image. Let’s create a simple Python Flask application inside a Docker container.

Step 1: Create a Simple Flask App

Create a directory and add the following Python script as app.py:

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello, Docker!"

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

Step 2: Create a Dockerfile

Inside the same directory, create a file named Dockerfile (no file extension) and add the following:

# Use the official Python image as the base
FROM python:3.9-slim

# Set the working directory inside the container
WORKDIR /app

# Copy the application files into the container
COPY app.py .

# Install Flask (required to run the app)
RUN pip install flask

# Expose port 5000 to the host
EXPOSE 5000

# Command to run the application
CMD ["python", "app.py"]

Step 3: Build and Run the Docker Container

1️⃣ Build the Docker image

docker build -t flask-app .

2️⃣ Run the container

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

3️⃣ Access the application
Open your browser and visit:

👉 http://localhost:5000

You should see "Hello, Docker!" displayed.