Dockerfile: Building Your Own Images

Dockerfile — a text file with instructions for building an image. Each instruction becomes a layer.

Minimal Python Dockerfile

FROM python:3.11-slim

WORKDIR /app

COPY requirements.txt .
RUN pip install -r requirements.txt

COPY . .

CMD ["python", "app.py"]

Core Instructions

FROM — base image. Always the first line.

FROM python:3.11-slim
FROM ubuntu:22.04
FROM scratch  # empty image

WORKDIR — working directory inside the container. Created automatically.

WORKDIR /app

COPY — copy files from the host into the image.

COPY app.py .          # copy a file
COPY . .               # copy everything
COPY src/ /app/src/    # copy a directory

RUN — run a command at build time. The result is saved into the image.

RUN pip install flask
RUN apt-get update && apt-get install -y curl

CMD — default command when the container starts.

CMD ["python", "app.py"]
CMD ["gunicorn", "app:app", "--bind", "0.0.0.0:8000"]

EXPOSE — documents the port (does not actually publish it, just informational).

EXPOSE 8000

ENV — environment variable.

ENV PYTHONDONTWRITEBYTECODE=1
ENV PYTHONUNBUFFERED=1

Building and Running

# Build an image with a tag
docker build -t myapp:v1 .

# Build with a different Dockerfile
docker build -f Dockerfile.prod -t myapp:prod .

# Run
docker run -p 8000:8000 myapp:v1

.dockerignore

The .dockerignore file at the project root works like .gitignore for Docker. It excludes files from the build context.

.venv/
__pycache__/
*.pyc
.git/
.env
tests/
*.md

Without .dockerignore, Docker copies everything into the image including .git and node_modules — builds will be slow and images will be bloated.

CMD vs ENTRYPOINT

# CMD — can be overridden at runtime:
CMD ["python", "app.py"]
# docker run myapp python other.py  ← replaces CMD

# ENTRYPOINT — fixes the command; CMD becomes its arguments:
ENTRYPOINT ["python"]
CMD ["app.py"]
# docker run myapp other.py  ← runs python other.py