INTRODUCTION TO DOCKER

A Dockerfile is a text file that contains a set of instructions used to automate the creation of a Docker image. It defines the environment, software dependencies, configurations, and commands that will be executed inside a container when it is built and run. Dockerfiles are used by Docker to build images, which can then be used to create containers.

Here's a breakdown of common Dockerfile instructions:

1. FROM: Specifies the base image to use (e.g., FROM ubuntu:20.04).
2. RUN: Executes commands inside the image, such as installing software packages (e.g., RUN apt-get update).
3. COPY: Copies files or directories from your local filesystem to the image (e.g., COPY . /app).
4. ADD: Similar to COPY, but can also handle URLs and unpack compressed files.
5. WORKDIR: Sets the working directory for subsequent instructions (e.g., WORKDIR /app).
6. CMD: Specifies the default command to run when a container is started (e.g., CMD ["python", "app.py"]).
7. EXPOSE: Informs Docker that the container will listen on specific ports (e.g., EXPOSE 80).
8. ENV: Sets environment variables in the container (e.g., ENV APP_ENV=production).
9. ENTRYPOINT: Defines the command that will always run when the container starts, even if a different command is provided at runtime.
10. VOLUME: Creates a mount point for volumes, allowing data persistence (e.g., VOLUME /data).

Example Dockerfile:

# Use the official Python image as the base image
FROM python:3.8-slim

# Set the working directory in the container
WORKDIR /app

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

# Install any necessary dependencies
RUN pip install -r requirements.txt

# Expose port 5000
EXPOSE 5000

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

Key Uses of a Dockerfile:

• Reproducibility: The same Dockerfile can be used to build the same environment across different systems, ensuring consistency.
• Automation: A Dockerfile allows you to automate the process of building an image with specific configurations and software.
• Portability: Once built, Docker images can be shared and run on any platform that supports Docker.

In essence, a Dockerfile is a blueprint for creating Docker images, ensuring the environment is consistent and reproducible.