Writing Docker File

A Dockerfile is a blueprint used to create a Docker image. It defines the base operating system image, required libraries and dependencies, source code, and other configurations. Dockerfiles use a Domain-Specific Language (DSL) and contain step-by-step instructions for building a Docker image. it should be create with filename Dockerfile without any extensions.

Docker builds image by reading the instructions from a Dockerfile. The Dockerfile instructions syntax is defined by the specification reference in the Dockerfile reference.Here are the most common types of instructions:

Instruction	Description
FROM <base-image>	Sets the base image for your Docker image. Every image starts from a base, like Ubuntu or Python.
RUN <commands>	Executes commands in a new layer on top of the current image and commits the results. Can be written in shell form or exec form.
COPY <src> <dest>	Copies files or directories from your local machine into the image.
WORKDIR <path>	Sets the working directory inside the container. All subsequent commands run from this location.
ENV <key>=<value>	Defines environment variables inside the container.
EXPOSE <port>	Documents which port the container listens on at runtime.
CMD ["executable"]	Specifies the default command to run when the container starts.

Example: Dockerfile for a Flask Web App

Scenario: We want to create a Flask web app using Ubuntu as the base image and run the application on port 8000.

Source code (hello.py):

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello Comrades! Welcome to the Docker world"

The following Dockerfile creates a container image, which includes all the dependencies needed and automatically starts your application.

#defining the base image for web app 
FROM ubuntu:22.04

# Install app dependencies
RUN apt-get update && apt-get install -y python3 python3-pip \
    && rm -rf /var/lib/apt/lists/*

# Install Flask
RUN pip3 install "flask==3.0.*"

# Copy application code
COPY hello.py /app/hello.py
WORKDIR /app

# Final configuration
ENV FLASK_APP=hello.py
EXPOSE 8000

CMD ["flask", "run", "--host=0.0.0.0", "--port=8000"]

Building docker image

we can build the docker image of the above Dockerfile using the command syntax: docker build -t <imagename:version> path

docker build -t test:latest .

Running images to Create container

we can run images to create a container , we can give a custom name to container or it can use random name by default.

docker run [options] <container-name>

running image to create a container

web app running on docker