Summary - Dockerfile

Combine FROM, RUN, COPY and CMD to write a Dockerfile.

Summary: the Dockerfile

In this group:

A Dockerfile describes an image with instructions from top to bottom; the first instruction is FROM.
The base image is pinned by version; -slim/-alpine reduce the size.
Each RUN is a layer; installing and cleaning up happen in the same RUN.
COPY copies from the context; the dependency file is copied before the code.
CMD is the default command, ENTRYPOINT the fixed program; the exec form is preferred.

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
EXPOSE 8000
CMD ["python", "app.py"]

Exercise A complete Python Dockerfile

Write a Dockerfile based on python:3.12-slim with working directory /app: copy requirements.txt and run pip install --no-cache-dir -r requirements.txt; copy the whole project; document port 8000; CMD is python app.py in exec form.

Show example solution

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
EXPOSE 8000
CMD ["python", "app.py"]

Test yourself

What is the drawback of a shell-form CMD?
- It produces a bigger image
- It can't use environment variables
- It slows the build
- The application may not receive SIGTERM and can't shut down cleanly
Show answer

The application may not receive SIGTERM and can't shut down cleanly — In shell form PID 1 is /bin/sh.
Set the working directory.
```
____ /app
```
Show answer
```
WORKDIR /app
```
Match.

FROM

First instruction
RUN

Build layer
EXPOSE

Port documentation
CMD

Default command

Next lesson: docker build

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