Docker · Lesson 4 of 8

Your First Dockerfile

Package your own app.

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Running other people's images is useful, but the real power is packaging your own app. A Dockerfile is a plain text recipe: start from a base image, copy your code in, install dependencies, and say how to start the app. Let's containerise a tiny Python web app.

The app

app.pypython

from flask import Flask

app = Flask(__name__)

@app.get("/")
def home():
    return {"message": "Hello from a container!"}

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

requirements.txttext

flask==3.0.3

host="0.0.0.0" matters: inside a container, 127.0.0.1 would only be reachable from the container itself.

The Dockerfile

Each line is an instruction. Docker runs them top to bottom and saves each one as a layer, so unchanged steps are cached and rebuilds are fast. That is why we copy requirements.txt and install packages before copying the rest of the code.

Dockerfiledockerfile

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
EXPOSE 5000
CMD ["python", "app.py"]

Instruction	Meaning
FROM	the base image to start from
WORKDIR	the folder inside the image to work in
COPY	copy files from your computer into the image
RUN	run a command while building
EXPOSE	document the port the app listens on
CMD	the command that starts the container

Build and run

terminal

docker build -t hello-app .
[+] Building 12.4s (10/10) FINISHED
 => [1/5] FROM python:3.12-slim
 => [3/5] RUN pip install --no-cache-dir -r requirements.txt
 => naming to docker.io/library/hello-app
docker run -d --name hello -p 5000:5000 hello-app
a91c03f5be72...
curl localhost:5000
{"message":"Hello from a container!"}

Keep images small and builds fast

.dockerignoreexclude .git, venv and caches from the build

Slim base imagespython:3.12-slim instead of python:3.12

Order layersdependencies first, code last, for better caching

One process per containerrun the database in its own container

.dockerignoretext

.git
__pycache__/
.venv/
*.pyc

Check your understanding

Quick check

Why copy requirements.txt before the rest of the code?

Summary

Key takeaways

A Dockerfile is the recipe for an image
docker build -t NAME . builds it
Order instructions so layers cache well

Next up: Volumes: Keep Your Data — data that survives containers

❮ Images & Containers Volumes: Keep Your Data ❯