Mahmoud Abas, Full-Stack Software Engineer & AI

Operating Systems 2 - Lab 08: Introduction to Docker - Containers and Images

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Operating Systems II - Lab 08

Introduction to Docker: Containers and Images

Why Docker exists, writing a Dockerfile, and building and running containers

The Problem Docker Solves

A project stack (say Laravel plus PostgreSQL/PostGIS plus Redis plus Angular) requires every developer to manually install compatible versions of everything
Version mismatches between components cause installation errors that can take hours or days to resolve
The same manual setup repeats at every environment, and each step is a new failure point

Local

manual install

new failure point

Test

manual install again

new failure point

Staging

manual install again

new failure point

Production

manual install again

What Is Docker?

Docker packages an application with everything it needs, runtime, dependencies, configuration, into a portable unit called a container
The container moves between environments with a single command, no reinstalling required
New developer setup becomes just two commands

bash

docker build -t myapp .
docker run myapp

The entire setup for a new developer.

Image vs Container

OOP Concept	Docker Equivalent
Class / Blueprint	Docker Image
Object / Instance	Docker Container

Docker Image: A read-only template built from a Dockerfile.

Docker Container: A running instance of that image, an isolated environment where the app actually executes.

Docker Image

docker run

Container 1

Container 2

Container 3

Image Layers and Caching

Every Dockerfile instruction creates a layer in the resulting image
Docker caches layers and reuses them if nothing in that layer, or before it, changed
Unchanged layers make subsequent builds significantly faster

node:14 base image

WORKDIR /app

COPY package.json

RUN npm install

COPY source files

Docker Hub

Docker Hub (hub.docker.com) is the public registry for Docker images, the GitHub of Docker images
Official base images exist for Node.js, PHP, Python, PostgreSQL, Redis, nginx, and more
Images carry multiple tags: full image, -slim, -alpine; Alpine variants are dramatically smaller, about 5 MB base

Writing a Dockerfile

Instruction	Purpose
`FROM`	Sets the base image
`WORKDIR`	Sets the working directory inside the container
`COPY`	Brings files in
`RUN`	Executes a build-time command
`CMD`	The startup command
`EXPOSE`	Documents the listening port

dockerfile

FROM node:14
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
EXPOSE 4000
CMD ["npm", "start"]

Building the Image

Copy package.json and run npm install before copying the rest of the source
This way Docker only reruns npm install when dependencies actually change, not on every code edit

bash

docker build -t express-node-app .
docker image ls

Build the image, then confirm it exists.

Running a Container

docker run starts a container from an image; add -d to run it detached in the background
Why isn't the app reachable from the host, even though EXPOSE 4000 is declared?

Containers run in an isolated network environment; a port is reachable from the host only once it is explicitly forwarded with -p

bash

docker run -d --name express-node-app-container -p 4000:4000 express-node-app

-p host:container maps a host port to the container's port, here 4000:4000

Managing Containers and Images

Command	Action
`docker ps`	Running containers
`docker ps -a`	All containers, including stopped
`docker stop <name>`	Stop a running container
`docker rm <name>`	Remove a stopped container
`docker rm -f <name>`	Force stop and remove in one command
`docker image ls`	List images
`docker rmi <image>`	Remove an image, only if no container uses it
`docker logs <name>`	Show a container's output

Choosing a Base Image

For production, prefer a smaller variant to reduce image size

dockerfile

FROM node:14-alpine

For development, or when full system tools are needed, the full image is acceptable
Compare sizes directly with docker image ls after rebuilding with each variant

Key Takeaways

Docker packages an app and its dependencies into a portable container, eliminating works on my machine setup problems
An Image is the read-only blueprint; a Container is a running instance of that image
Dockerfile layers are cached; order instructions from least to most frequently changing for faster rebuilds
Docker Hub hosts official base images, each with size and variant tags like -alpine
docker build creates the image; docker run -p host:container starts it and exposes it to the host
Essential lifecycle commands: docker ps, docker stop, docker rm, docker rmi, docker logs

Open this lesson

Mahmoud AbasLab 08: Introduction to Docker - Containers and Images

OOP Concept

Docker Equivalent

Class / Blueprint

Docker Image

Object / Instance

Docker Container

Instruction

Purpose

FROM

Sets the base image

WORKDIR

Sets the working directory inside the container

COPY

Brings files in

RUN

Executes a build-time command

CMD

The startup command

EXPOSE

Documents the listening port

Command

Action

docker ps

Running containers

docker ps -a

All containers, including stopped

docker stop <name>

Stop a running container

docker rm <name>

Remove a stopped container

docker rm -f <name>

Force stop and remove in one command

docker image ls

List images

docker rmi <image>

Remove an image, only if no container uses it

docker logs <name>

Show a container's output