Docker

Main components

1. Image

   • A blueprint or template

   • Read-only

   • prebuilt images are available in a public registry (DockerHub)

2. Container

   • A running instance of an image

   • Lightweight, fast, disposable

   • You can start, stop, delete it anytime

3. Dockerfile A recipe that tells Docker how to build an image

4. Docker Engine The actual software that builds images and runs containers

Install Docker & Docker Compose

On Ubuntu / Debian Linux

1. System update

sudo apt update
sudo apt upgrade -y

1. Install Docker

sudo apt install docker.io -y

1. Start Docker service

sudo systemctl start docker
sudo systemctl enable docker

Check Docker and Docker Compose installed versions:

docker --version
docker compose version

Run Docker without sudo

sudo usermod -aG docker $USER

• Log out and log back in, then test:

docker run hello-world

Install Docker Compose (Modern way)

Docker Compose is now built into Docker as a plugin.

If Docker Compose is missing:

sudo apt install docker-compose-plugin -y

macOS

Install Docker Desktop for Mac

Windows

1. Install Docker Desktop

2. Enable WSL 2

Dockerfile

Plain text file that contains instructions telling Docker how to build an image.

The instructions are usually in this order:

• FROM

• WORKDIR

• COPY / ADD

• RUN

• EXPOSE

• CMD / ENTRYPOINT

FROM

FROM python:3.11

• Base image (e.g. python:3.11)

• Every Dockerfile starts with this (except scratch)

• Example: Ubuntu, Node, Java, Python

WORKDIR

WORKDIR /app

• Sets the working directory inside the container

• Like cd /app

• Recommended instead of RUN cd ...

COPY

COPY . .

• Copies files from your machine into the image

• First path = host

• Second path = container

ADD

ADD app.tar.gz /app

• Similar to COPY, but more powerful

• Can extract archives

• Can download URLs

RUN

RUN pip install flask

• Executes commands at build time

• Creates a new image layer

• Used for installing dependencies

EXPOSE

EXPOSE 5000

• Documents which port the app uses

• Does NOT publish the port

• Used mainly for readability and tools

ENV

ENV ENVIRONMENT=production

• Sets environment variables

• Available inside the container

CMD

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

• Default command when container starts

• Can be overridden with docker run

ENTRYPOINT

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

• Fixed command

• Arguments can be appended

• Harder to override than CMD

Full simple example

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
EXPOSE 5000
CMD ["python", "app.py"]

Build & Run

• Move to the folder containing the application and Dockerfile and build the image with <image_name> name

docker build -t <image_name> .

• Run the container

docker run <image_name>

• Stop the container

docker stop <image_name>

Run command syntax

docker run [OPTIONS] IMAGE [COMMAND] [ARGS...]

Flag	Purpose	What it does	Example
-d	Detached mode	Runs container in the background	docker run -d nginx
-it	Interactive terminal	-i keeps STDIN open, -t allocates a TTY (great for shells/debugging)	docker run -it ubuntu bash
--name	Container name	Assigns a custom name to the container	docker run --name my-nginx nginx
-p	Port mapping	Maps host port → container port	docker run -p 8080:80 nginx
-v	Volumes / bind mounts	Persists data or shares files between host and container	docker run -v mydata:/data myapp
-v	Bind mount	Mounts a host directory into the container	docker run -v /host/path:/container/path myapp
-e	Environment variables	Sets env vars inside the container	docker run -e DB_HOST=localhost myapp
-e (multiple)	Multiple env vars	Passes multiple env vars	docker run -e A=1 -e B=2 myapp
--rm	Auto-remove	Deletes container after it exits (one-off runs)	docker run --rm busybox echo "hi"
--restart	Restart policy	Controls container restart behavior (e.g. no by default, on-failure, always, unless-stopped)	docker run --restart unless-stopped nginx
--network	Networking	Connects container to a custom network	docker run --network mynet myapp
--env-file	Env vars from file	Loads env vars from a file (keeps secrets out of CLI history)	docker run --env-file .env myapp
--cpus	CPU limit	Limits CPU usage	docker run --cpus="1.5" myapp
--memory	Memory limit	Limits RAM usage	docker run --memory="512m" myapp
--user	Run as non-root	Runs container as a specific UID/GID	docker run --user 1000:1000 myapp
-w, --workdir	Working directory	Sets working directory inside container	docker run -w /app myimage
--entrypoint	Override entrypoint	Replaces image entrypoint (useful for debugging)	docker run --entrypoint bash myimage