Dockerfile Basics
A Dockerfile is a script containing a set of instructions to automate the creation of Docker images. It defines everything needed to run an application, including:
✔ Base Image – The starting point (e.g., Ubuntu, Python, Node.js).
✔ Dependencies – Libraries and packages required by the app.
✔ Configurations – Environment variables, file paths, ports, etc.
✔ Execution Commands – How the application should start.
Why Use a Dockerfile?
✅ Automates the image creation process.
✅ Ensures consistency across different environments.
✅ Makes application deployment easier and faster.
Basic Structure of a Dockerfile
A simple Dockerfile looks like this:
# 1️. Specify the base image
FROM python:3.9
# 2️. Set the working directory
WORKDIR /app
# 3️.Copy application files
COPY . /app
# 4️. Install dependencies
RUN pip install -r requirements.txt
# 5️. Expose a port (optional)
EXPOSE 5000
# 6️. Define the command to run the app
CMD [“python”, “app.py”]
Understanding Dockerfile Instructions
1️. FROM – Specify the Base Image
Defines the starting point for your application.
Example:
FROM ubuntu:latest
or
FROM node:16
2️. WORKDIR – Set the Working Directory
Defines the folder where commands will run inside the container.
WORKDIR /app
3️. COPY – Copy Files into the Container
Copies application files from the host to the container.
COPY . /app
4️. RUN – Execute Commands Inside the Image
Used to install dependencies or configure the system.
RUN apt-get update && apt-get install -y curl
or
RUN pip install -r requirements.txt
5️. EXPOSE – Define the Application Port
Opens a port for communication.
Dockerfile
EXPOSE 8080
6️. CMD – Define the Default Command
Specifies what the container should do when it starts.
CMD [“node”, “server.js”]
or
CMD [“python”, “app.py”]
Building and Running an Image from a Dockerfile
1️. Build the Image
Run the following command in the folder where the Dockerfile is saved:
docker build -t my_app .
2️. Run a Container from the Image
docker run -d -p 5000:5000 my_app
Now your application is running inside a container!
Conclusion
• A Dockerfile is a script that automates the creation of Docker images.
• It defines dependencies, configurations, and execution commands.
• Using docker build and docker run, you can create and deploy containers easily.
