This content originally appeared on DEV Community and was authored by Aisalkyn Aidarova

1. What is Docker? (Very Simple)

Before Docker:
Application depends on OS, packages, libraries → “it works on my machine” problem.

Docker solves it by packaging:

App Code + Dependencies + Runtime = IMAGE

Then you run containers from that image.

2. Docker Architecture

+--------------------+
|    Docker Client   |  -> docker commands
+--------------------+
          |
+--------------------+
|   Docker Engine    |  (daemon)
+--------------------+
          |
+--------------------+
| Images | Containers |
+--------------------+

3. Basic Docker Commands

Check Docker version:

docker --version

See running containers:

docker ps

See all containers (including stopped):

docker ps -a

See images stored locally:

docker images

4. Run Your First Container

docker run -it ubuntu bash

Means:

• -it → interactive terminal
• ubuntu → official image
• bash → open shell

Exit:

exit

5. Run a Web App (Nginx)

docker run -d -p 8080:80 nginx

• -d → run in background
• -p 8080:80 → expose port

Open browser:

http://localhost:8080

6. Stop & Remove Containers

Stop:

docker stop <container_id>

Remove:

docker rm <container_id>

Remove unused:

docker system prune -a -f

7. Create Your Own Docker Image

Step 1: Create app.py

from flask import Flask
app = Flask(__name__)

@app.route("/")
def home():
    return "Hello from Docker!"

app.run(host="0.0.0.0", port=5000)

Step 2: Create requirements.txt

flask

Step 3: Create Dockerfile

FROM python:3.11-slim

WORKDIR /app

COPY requirements.txt .
RUN pip install -r requirements.txt

COPY . .

CMD ["python", "app.py"]

Step 4: Build the image

docker build -t myflaskapp .

Step 5: Run container

docker run -p 5000:5000 myflaskapp

Open:

http://localhost:5000

You just built your own containerized application

8. Docker Volumes (Persistent Data)

Without volumes, container data disappears when container stops.

Create a named volume:

docker volume create data1

Run container with persistent storage:

docker run -d -p 3307:3306 -e MYSQL_ROOT_PASSWORD=pass -v data1:/var/lib/mysql mysql

Now database data is permanent.

9. Docker Networks (Connect Multiple Containers)

docker network create mynet

Backend:

docker run -d --network=mynet --name=db postgres

Frontend:

docker run -d --network=mynet --name=web nginx

They can now talk to each other using container name.

10. Docker Compose (Run Multi-Container App)

Create docker-compose.yml:

version: "3.8"
services:
  web:
    image: nginx
    ports:
      - "8080:80"
  redis:
    image: redis

Run:

docker compose up -d
docker compose ps
docker compose down

11. Production Best Practices (Very Important for Interview)

12. Real DevOps Interview Answer

“Docker allows us to package applications with dependencies into container images that run consistently across environments. We write Dockerfiles to build images and run containers using Docker Engine. Using volumes ensures persistent storage, and custom networks allow multi-service communication. In production we use Docker with Kubernetes, OPA Gatekeeper for policy enforcement, and CI/CD pipelines for image delivery.”

This content originally appeared on DEV Community and was authored by Aisalkyn Aidarova