What is a Dockerfile?

Learn that a Dockerfile is a recipe for an image.

What is a Dockerfile?

A Dockerfile is a plain text file describing step by step how an image is built. Each line is an instruction (written in UPPERCASE) and its arguments. Docker runs the instructions from top to bottom:

FROM python:3.12-slim
WORKDIR /app
COPY app.py .
CMD ["python", "app.py"]

This file is written once, put under version control, and everyone produces the same image with docker build.

Overview of instructions

FROM: base image
WORKDIR: working directory
COPY: copying files
RUN: running a command during the build
ENV: environment variable
EXPOSE: the port the application listens on (documentation)
CMD: the default command run when the container starts

# comments start with #
FROM nginx:alpine
COPY index.html /usr/share/nginx/html/
EXPOSE 80

Exercise The first Dockerfile

Write a Dockerfile for a static site: base image nginx:alpine; copy index.html into /usr/share/nginx/html/; document port 80.

Show example solution

FROM nginx:alpine
COPY index.html /usr/share/nginx/html/
EXPOSE 80

Test yourself

What must the first instruction of a Dockerfile be (apart from comments and ARG)?
- FROM
- RUN
- CMD
- COPY
Show answer

FROM — Every image is built on top of a base image.
Write the instruction documenting the port the application listens on.
```
____ 80
```
Show answer
```
EXPOSE 80
```
Match the instructions with their jobs.

FROM

Base image
COPY

Copies files
RUN

Command during build
CMD

Start command

Next lesson: The FROM instruction

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