Sadiul Hakim

Posted on Oct 14

Docker Tutorial for beginner

#docker

1. What is Docker?

Docker is a platform that lets you package your applications (code + dependencies + configuration) into containers — lightweight, isolated environments that run consistently anywhere (on your machine, on a server, or in the cloud).

Think of it like this:

Without Docker: “It works on my machine!”
With Docker: “It works the same everywhere!”

It uses OS-level virtualization, not full virtual machines — so containers are fast and lightweight.

2. Why use Docker?

Problem	Docker Solution
Different dev/prod setups	Same container image works everywhere
Dependency conflicts	Each container has isolated environment
Complex setup	Just `docker run postgres` and it works
Scaling	Containers can be easily replicated
CI/CD	Build once → deploy anywhere

3. When to Use vs. When Not To Use Docker

Use Docker When:

You want a reproducible environment.
You need to run multiple services (DB, API, frontend) locally.
You deploy microservices or cloud-native apps.
You want simple dev/test setups.

Avoid Docker When:

You only run a single, simple app (no dependencies).
You need a full OS or GUI apps (containers are headless).
You’re working in highly resource-limited environments.
You need ultra-low-level hardware access.

4. Image vs. Container

Term	Description	Analogy
Image	A blueprint — immutable snapshot of an environment	A “class” or “template”
Container	A running instance of an image	An “object” created from a class

Relationship:
You create containers from images.
You can have multiple containers from the same image.

Example:

docker pull postgres
docker run -d --name db1 postgres
docker run -d --name db2 postgres

Here both db1 and db2 come from the same postgres image.

5. Basic Docker Commands Cheat Sheet

List things

# List all images
docker images

# List running containers
docker ps

# List all containers (including stopped)
docker ps -a

a. Pull an image (e.g. PostgreSQL)

docker pull postgres

This downloads the image from Docker Hub.

b. Create & run a container

docker run -d \
  --name my-postgres \
  -e POSTGRES_PASSWORD=secret \
  -p 5432:5432 \
  postgres

Explanation:

-d → detached (background)
--name my-postgres → give it a name
-e → set environment variable
-p 5432:5432 → map container port → host port
postgres → the image name

c. Reuse a container

# Stop
docker stop my-postgres

# Start again
docker start my-postgres

Docker containers can be restarted without losing data if you use volumes, otherwise data is wiped when removed.

d. Enter a container terminal

docker exec -it my-postgres bash
# or for Alpine-based images
docker exec -it my-postgres sh

-it means interactive terminal.

e. See container logs

docker logs my-postgres
docker logs -f my-postgres   # follow live logs

f. Remove a container

docker rm my-postgres
# If it’s running, first stop:
docker stop my-postgres && docker rm my-postgres

g. Remove an image

docker rmi postgres

If containers still use it, you’ll need -f (force):

docker rmi -f postgres

h. What is a Docker Network?

A Docker network connects containers together so they can talk to each other by name.

Default: all containers use the same bridge network.
You can create custom networks for isolation and internal DNS.

When to use:

When running multiple containers (e.g., API + DB).
When you don’t want to expose all ports to the host.

i. Create and connect containers to a network

# Create network
docker network create mynet

# Run containers on it
docker run -d --name db --network mynet postgres
docker run -d --name app --network mynet my-spring-app

Now your app container can connect to Postgres via db:5432 instead of localhost.

Bonus: Cleaning up

docker system prune          # Remove unused containers/images/networks
docker system prune -a       # Remove EVERYTHING unused

Summary Mindmap

Docker
├── Images (templates)
│   └── Created from Dockerfile or pulled from Docker Hub
├── Containers (instances of images)
│   ├── Start/stop/restart
│   ├── Exec into (bash)
│   └── Logs and volumes
├── Networks
│   └── Connect containers by name
└── Commands
    ├── docker images / ps / pull / run / stop / rm / logs
    └── docker network create / connect

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