Master Docker: From Basics to Dockerfile Best Practices

Core Concepts

Image : read‑only template that contains all instructions to create a container.

Container : a running instance of an image that can be started, stopped, or removed.

Repository : storage location for images (e.g., Docker Hub, private registries such as Harbor).

Installation

Linux (Ubuntu)

Remove old versions:

sudo apt-get remove docker docker-engine docker.io containerd runc

Install dependencies:

sudo apt-get update
sudo apt-get install apt-transport-https ca-certificates curl gnupg-agent software-properties-common

Add Docker GPG key:

curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

Add stable repository:

sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

Install Docker Engine:

sudo apt-get update
sudo apt-get install docker-ce docker-ce-cli containerd.io

Verify installation:

sudo docker run hello-world

Tip: To run Docker without sudo, add your user to the docker group:

sudo usermod -aG docker $USER
newgrp docker

Windows / macOS

Download Docker Desktop.

Run the installer and follow the wizard.

Start Docker Desktop.

Verify with docker --version and docker run hello-world.

Dockerfile Overview

A Dockerfile is a plain‑text script that defines a sequence of instructions to build an image.

Common Instructions

FROM : specify base image (e.g., FROM ubuntu:20.04).

RUN : execute a command during build (e.g., RUN apt-get update && apt-get install -y python3).

COPY : copy files from build context into the image (e.g., COPY . /app).

ADD : similar to COPY but supports URLs and automatic extraction (e.g., ADD https://example.com/file.tar.gz /tmp/).

WORKDIR : set working directory (e.g., WORKDIR /app).

ENV : set environment variables (e.g., ENV PATH /usr/local/bin:$PATH).

EXPOSE : declare listening port (e.g., EXPOSE 80).

CMD : command executed when container starts (single command, e.g., CMD ["python", "app.py"]).

ENTRYPOINT : fixed entrypoint that cannot be overridden by docker run arguments (e.g., ENTRYPOINT ["python"]).

ARG : build‑time variable (e.g., ARG version=latest).

Sample Dockerfile

# Base image
FROM python:3.8-slim

# Set work directory
WORKDIR /app

# Copy project files
COPY . /app

# Install dependencies
RUN pip install --no-cache-dir -r requirements.txt

# Expose port
EXPOSE 80

# Environment variable
ENV NAME World

# Startup command
CMD ["python", "app.py"]

Building and Running

# Build the image
docker build -t my-python-app .

# Run the container, mapping host port 4000 to container port 80
docker run -p 4000:80 my-python-app

Best Practices

Use a .dockerignore file to exclude unnecessary files from the build context.

Run a single main process per container.

Combine multiple RUN commands with && to reduce image layers.

Prefer official base images.

Avoid the latest tag; pin specific versions for reproducibility.

Leverage multi‑stage builds to minimize final image size.

Use volumes for persistent data instead of writing inside the container.