developer-roadmap/src/data/roadmaps/docker/content/106-building-container-images/index.md

Building Container Images

Container images are executable packages that include everything required to run an application: code, runtime, system tools, libraries, and settings. By building custom images, you can deploy applications seamlessly with all their dependencies on any Docker-supported platform.

Dockerfile

The key component in building a container image is the Dockerfile. It is essentially a script containing instructions on how to assemble a Docker image. Each instruction in the Dockerfile creates a new layer in the image, making it easier to track changes and minimize the image size. Here's a simple example of a Dockerfile:

# Use an official Python runtime as a parent image
FROM python:3.7-slim

# Set the working directory to /app
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install any needed packages specified in requirements.txt
RUN pip install --trusted-host pypi.python.org -r requirements.txt

# Make port 80 available to the world outside this container
EXPOSE 80

# Define environment variable
ENV NAME World

# Run app.py when the container launches
CMD ["python", "app.py"]

Visit the following resources to learn more:

• @official@Docker Image Builder
• @official@Dockerfile Reference
• @opensource@Dockerfile Examples