Docker Fundamentals – From Beginner to Senior DevOps Level (Part 1)
1️⃣ What is Docker?
Docker is a containerization platform that allows us to package an application along with its dependencies, libraries, and configuration into a single unit called a container.
Beginner understanding
An application needs OS libraries, runtime, and dependencies
Docker bundles everything together
The container runs the same everywhere
In simple terms, Docker makes applications portable and consistent.
2️⃣ Why Do We Need Docker?
Before Docker:
Application works on developer machine
Fails on test or production servers
Environment differences cause issues
With Docker:
Same container runs in dev, test, and prod
Faster deployments
Easy scaling and rollback
Docker solves the famous problem: "It works on my machine"
3️⃣ Docker vs Virtual Machines
This is one of the most common interview questions.
Virtual Machines (VMs)
Virtualize hardware
Each VM has its own OS
Heavy and slow to start
Docker Containers
Virtualize the operating system
Share the host OS kernel
Lightweight and fast
Virtual MachineDocker ContainerFull OSShares host OSHeavy (GBs)Lightweight (MBs)Slow startupStarts in seconds
Senior DevOps perspective
Docker uses OS-level virtualization, which makes it efficient but also requires strong kernel-level security.
4️⃣ Core Docker Concepts
Understanding these concepts clearly is very important before moving to advanced topics.
🔹 Docker Image
A Docker image is a read-only template used to create containers.
Contains application code
Contains dependencies and runtime
Does not run by itself
Examples:
nginximagenodeimageopenjdkimage
Think of an image as a blueprint.
🔹 Docker Container
A Docker container is a running instance of an image.
Created from an image
Has its own process
Has its own network and filesystem
If image is a class, container is an object.
Containers are ephemeral — when deleted, data inside is lost unless stored externally.
5️⃣ Docker Architecture (Simple and Clear)
Docker follows a client–server architecture.
Main Components
🔸 Docker Client
Used by users
Runs commands like
docker run,docker build
🔸 Docker Daemon (dockerd)
Runs in the background
Builds images
Runs containers
Manages networking and storage
🔸 Docker Registry
Stores Docker images
Can be public (Docker Hub)
Or private (ECR, ACR, private registry)
Docker Command Flow
Docker Client → Docker Daemon → Docker Registry → Container
Example:
You run
docker run nginxDocker daemon pulls image from registry
Container is created and started
Senior-level note
Internally, Docker uses containerd as the container runtime to manage container lifecycle.
6️⃣ Summary (Interview-Ready)
Docker is a containerization platform
Containers package app + dependencies
Docker is lightweight compared to VMs
Image is a template, container is a running instance
Docker follows client–server architecture
This foundation is critical before learning Dockerfile, networking, volumes, and Docker Compose.
📌 Next Post: Dockerfile, Image Layers, CMD vs ENTRYPOINT, and Best Practices
✨ If you found this useful, follow along for the next part of the Docker series.
