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Rutvik Makvana
Rutvik Makvana

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Docker Made Simple: From Zero to Docker with Real Interview Questions

#docker #backend #cloud #devops

Question - Why we need docker?

  • Imagine two developers:

    • Dev A builds an app on his laptop.
    • Dev B tries to run it on her laptop.

  • Dev A says:

    • It works perfectly on my machine!

  • Dev B runs it and gets 100 errors.

  • Why?

  • Because Dev A's computer has:

    • Node v18
    • Python 3.11
    • Some libraries
    • Special OS settings

  • Dev B's computer has different versions.

  • This problem existed for years.

  • Then came the hero ==> Docker

Question - What is Docker?

  • Docker is a containerization platform used to package an application and its dependencies into a lightweight container that runs consistently across environments.

  • Key benefits:

    • Environment consistency
    • Faster deployment
    • Isolation
    • Lightweight compared to VMs

Question - What is the difference between Docker and Virtual Machines?

  • Both provide isolated environments, but they work differently. 
  Feature          Docker Containers   Virtual Machines
  ---------------- ------------------- ------------------------
  OS               Share host OS       Each VM has its own OS
  Size             MBs                 GBs
  Startup Time     Seconds             Minutes
  Performance      Faster              Slower
  Resource Usage   Lightweight         Heavy
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  • Containers are more efficient because they share the host kernel.

Question - Main components of Docker:

  • Docker Client
  • Docker Daemon
  • Docker Images
  • Docker Containers
  • Dockerfile
  • Docker Registry (Docker Hub)

Docker Client

  • The Docker Client is the tool developers use to interact with Docker.
  • When you run commands like: 
docker build
docker run
docker pull
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  • You are using the Docker Client.
  • The client sends these commands to the Docker Daemon.

Docker Daemon

  • The Docker Daemon (dockerd) is the background service that manages everything.
  • It is responsible for:
    • Building images
    • Running containers
    • Managing networks
    • Managing volumes
  • The Docker client communicates with the daemon to perform actions.

Docker Image

  • A Docker Image is a read-only template used to create containers.
  • It contains:
    • Application code
    • Runtime (Node, Python, etc.)
    • Libraries
    • Dependencies
    • Environment setup
  • Example image: 
docker pull nginx
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Docker Container

  • A Docker Container is a running instance of an image.
  • Think of it like: 
Image = Blueprint
Container = Running Application
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  • Containers are:
    • Lightweight
    • Fast
    • Isolated

Dockerfile

  • A Dockerfile is a script that defines how to build a Docker image.
  • Example: 
FROM node:18
WORKDIR /app
COPY . .
RUN npm install
CMD ["npm","start"]
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  • This tells Docker how to build the image step by step.

Docker Registry

  • A Docker Registry is a place where Docker images are stored and shared.
  • The most popular public registry is: Dcoker Hub
  • Example Command: 
docker pull ubuntu
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  • This downloads the image from Docker Hub.

Question - Most Important Dockerfile Commands 

FROM           Base image
WORKDIR        Set working directory
COPY           Copy files
ADD        Copy + extract
RUN        Execute commands
CMD        Default command
ENTRYPOINT     Main executable
EXPOSE         Declare port
ENV        Environment variable
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Question - Difference Between CMD and ENTRYPOINT 

CMD                 ENTRYPOINT
Default command         Fixed executable
Can be overridden   Harder to override
Used for parameters Used for main command
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  • Example: 
CMD ["node", "server.js"]
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Question - What is .dockerignore?

  • Similar to .gitignore, it prevents unnecessary files from being copied to the Docker image.
  • Example: 
node_modules
.git
.env
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Question - How to Build a Docker Image? 

docker build -t my-node-app .
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Question - How to Run a Container? 

docker run -p 3000:3000 my-node-app
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Question - How to See Running Containers? 

docker ps
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  • All containers: 
docker ps -a
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Question - How to Run a Container? 

docker stop container_id
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Question - What is Docker Compose?

  • Docker Compose is a tool used to define and run multi-container Docker applications using a docker-compose.yml file.
  • Example use:
    • Node.js
    • MongoDB
    • Redis
  • All run together

Question - Example docker-compose.yml for Node.js + Mongo 

version: "3"

services:
  app:
    build: .
    ports:
      - "3000:3000"
    depends_on:
      - mongo

  mongo:
    image: mongo
    ports:
      - "27017:27017"
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Question - Important Docker Compose Commands

  • Start services 
docker-compose up
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  • Background mode 
docker-compose up -d
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  • Stop services 
docker-compose down
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  • Rebuild 
docker-compose up --build
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Question - What is depends_on in Docker Compose?

  • It defines service dependency order.
  • Example : 
depends_on:
  - mongo
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  • Meaning Node starts after Mongo container.

Question - What is build vs image?

  • build
    • Builds image from Dockerfile
    • Used for custom apps
  • Example : 
build: .
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  • image
    • Uses existing image
    • Used for prebuilt apps
  • Example : 
image: mongo
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Question - What is a Docker Volume?

  • A Docker Volume is used for persistent data storage outside the container.
  • Normally when a container stops or is deleted, data is lost.
  • Volumes solve this problem.
  • Example : 
docker run -v myvolume:/data nginx
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  • Common use cases :
    • Databases
    • Logs
    • File uploads

Question - What is Bind Mount in Docker?

  • A Bind Mount connects a host machine directory to a container directory.
  • Example : 
docker run -v /host/folder:/container/folder nginx
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  • Difference from Volume: 
  Volume                  Bind Mount
  ----------------------- -----------------
  Managed by Docker       Managed by host
  Stored in Docker area   Any host path
  Portable                Less portable
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Question - What is Docker Network?

  • Docker Network allows containers to communicate with each other.
  • Types of networks: 
  Network Type   Description
  -------------- --------------------------------
  bridge         Default network for containers
  host           Container shares host network
  none           No networking
  overlay        Used in Docker Swarm
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  • Example: 
docker network create mynetwork
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Question - What is Docker Layer Caching?

  • Docker images are built in layers.
  • Each command in Dockerfile creates a new layer.
  • Example: 
FROM node:18 
WORKDIR /app 
COPY package.json . 
RUN npm install 
COPY . .
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  • If package.json doesn’t change, Docker reuses the cached layer.
  • Benefits:
    • Faster builds
    • Reduced rebuild time

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