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Dockerfiles

A Dockerfile is a plain text file that serves as a blueprint or "recipe" for creating a Docker container image. It contains a sequential list of instructions that Docker executes to assemble your application environment, dependencies, and configuration into a portable, consistent package.

Dockerfile Example

Core Instructions​

Every line in a Dockerfile is an instruction. Here are the most fundamental ones:

  • FROM: Sets the base image (e.g., FROM python:3.13 or FROM node:18). This must be the first instruction.
  • WORKDIR: Sets the working directory inside the container where subsequent commands will run.
  • COPY: Copies files from your local host machine into the container image.
  • RUN: Executes commands (like installing packages) during the build process.
  • ENV: Sets environment variables for the container.
  • EXPOSE: Documents which port the container listens on.
  • CMD: Specifies the default command to run when the container starts.

Example: A Simple Python App​

# Use an official base image
FROM python:3.13

# Set the working directory
WORKDIR /app

# Copy dependency file and install
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the application code
COPY . .

# Specify the command to run the app
CMD ["python", "app.py"]