# devops / docker / dockerfile

The Recipe of Docker: Dockerfiles

A Dockerfile is like a recipe card 🍰. It tells Docker how to build an image step by step. Each line is an instruction, and Docker reads it from top to bottom to bake your final container image.

Learn the Instructions →Building & Pushing
$ docker build -t my-app .✓
Step 1/4FROM ubuntu:latest
Step 2/4RUN apt-get update
Step 3/4COPY code /app
Step 4/4CMD ["nginx"]
StatusSuccessfully built
Top-Down
Execution
Strict
Case-Sensitive
# analogy

Baking the Cake

Think of building an image just like baking a cake. You can't bake it before you add the flour. Docker follows the exact same logical, sequential progression.

🧁 The Cake Recipe

Take flour → Add sugar → Mix → Bake in oven.

🐳 The Dockerfile

FROM ubuntu → RUN apt-get update → COPY code → CMD run app

FUN FACT

Dockerfile instructions are strictly case-sensitive! Writing FROM ubuntu ✅ works perfectly, but writing from ubuntu ❌ will fail the build.

# instructions

Dockerfile Instruction Dictionary

DockerfileEXAMPLE
1 2 3 4 5 6 7 8 9 10 11
FROM ubuntu:latest
LABEL version="1.0"
ENV owner="shan"
VOLUME ["/data"]
WORKDIR /lak

COPY AStc /lak
ADD dumptar /lak
RUN apt update && useradd -ms /bin/bash shan
USER shan
EXPOSE 8080
CMD ["ping", "8.8.8.8"]

What they do

  • FROM

    Defines the base image. Every valid Dockerfile must start with this.

  • LABEL / ENV

    LABEL adds metadata (like version). ENV sets persistent environment variables inside the image.

  • WORKDIR

    Sets the default directory for all subsequent commands (like cd).

  • COPY / ADD

    Both copy files from host to image. ADD has extra features like unzipping or downloading from URLs.

  • RUN

    Executes commands during the build time (e.g., installing packages).

  • CMD / ENTRYPOINT

    Defines the default command to run when the container starts. (Only one CMD is allowed).

# execution

Foreground vs Background

A container must have a foreground process to keep running. If there is no foreground process holding the terminal open, the container completes its task and stops immediately.

keeps container alive

Foreground Process

The main task. For example, running an Apache or Nginx server directly in the terminal. As long as the server runs, the container lives.

helper tasks

Background Process

Tasks running silently (like a cron job or background logging). If your CMD only starts background tasks, the container will instantly exit.

# workflow

Building & Pushing Images

Tags are version labels for images (e.g., app:v1). Think of it like saving a file as Essay_v1.docx. If no tag is specified, Docker automatically applies the latest tag.

THE WORKFLOW

Build → Save the project on your laptop.
Push → Upload it to Google Drive (Docker Hub).

terminal
# 1. Build the Image
# (The period '.' means use the Dockerfile in current directory)
$ docker build -t username/app:1.0 .
Successfully built 8a9b2c3d4e5f
Successfully tagged username/app:1.0
 
# 2. Tag an existing image (optional)
$ docker tag 8a9b2c3d4e5f username/app:v1
 
# 3. Push to Docker Hub
$ docker push username/app:1.0
The push refers to repository [docker.io/username/app]
1.0: digest: sha256:7b8c... size: 1532
$