Docker Zero to Hero

What is Docker?

• Docker is a software platform that allows you to build, test, and deploy applications quickly.
• Docker is an open source, which helps to deploy apps on containers instead of using virtual machines

Difference between Docker and VM

• Technically Docker used Host OS resources to run applications, however VM have it's own OS and Kernel.
• Docker is good for deploying app fast and accessible from anywhere, because when you create Docker image, you can push into artifacts like docker hub, ECR or wherever you want, then you can pull and create containers.
• In VM, case, it is not easy but possible with VM images, but again it is not easy!
• Docker used less resources as compare to use VMs, so if I want to run an application, I will just focused on application arhitecture.

Technical Comparison

I launched Ubuntu in container and you can see, it is only taking 4mbps RAM from my Host OS, if I used VM, it is required at least 1.5 GB RAM to run Ubuntu. So, docker only scale the resource whenever it is required, as compare to VMs, you need to define the resources even if it is not in used, you will have to assign those resources.

Lets learn how to use Docker.

If you are using Linux as a HostOS, then you already have it for Windows, you need to install docker desktop.

You might have question, why we need to install Docker Desktop?

So, Docker as technology can only be run on Linux OS. Docker Desktop does 2 things:

1. Creates Linux VM(WSL) on your host OS (Windows / Mac)
2. Technically Windows Kernel is different then Linux, to solve that problem, you need a platform to make it work, so Docker Desktop create Linux kernel, to handle I/O from user and give the env to run container.

Lets install docker and run first container.

• Installation Guide - doc.docker.com
• Once you have installed, you can launch docker desktop or run docker command in cli.
• I would also recommend, you can create account in docker hub, so that later on we can publish our docker images and use it from anywhere.

Docker CLI

• You can launch CMD, and run Docker --help help command, have all other commands reference which we will going to cover.

Docker Container Vs Docker Image.

Lets deep dive into Docker container and image Most of the people don't understand the difference between docker container and image.

• Containers create with Docker Images.
• Docker images are built with docker instructions, the instructions contains application requirements and dependency.
• Docker image is the template loaded onto the container to run it, like a set of instructions. You store images for sharing and reuse, but you create and destroy containers over an application's lifecycle.

Dockerfile: it is a set of instructions for creating image, lets say I want to create an application in python, then I want to containerized that application, so I will write my application dependencey and built docker image, with docker image, I can run 'N' number of containers.

We will create Dockerfile later on but to learn fast, we will use available images from docker hub.

Visit the docker hub - hub.docker.com

• These official images useful for reuse instead of writing dockerfile. lets say I needed Nginx Server for my Web App, I can use Nginx image directly by using pull command.

For example: docker pull "image-name"

Lets pull nginx image and create our first container

docker pull nginx

If you want to check your images, run below command.

docker images

So, we have image but we need to create container with image

For example: docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

Lets create container.

docker run -d -p 8080:80 --name=my-nginx nginx

Now you can access Nginx, with https://localhost:8080

Let's break down above command

• docker run: when you want to run first time container with docker image.
• -d: detech the container from terminal, and print the container id.
• -p: Specify the port, as you see I mentioned 8080:80
  • Containers are isolated from your network, so If you want to expose the container, then you need to bind container port with HostOS port.
  • So, you are saying I am binding my hostOS port with container port and similar way, you are discribing HOST-PORT:CONTAINER-PORT.
  • Make sure, you must know about the container port where your application is running and hostOS port also, in case you are not able to access your application via specific port, then check your hostOS if that port is available or not.
• --name: You can specify the application name, it is optional.
• image: Mentioned the image.

There are more arguments available on Docker official website, you may read the documentation -- > docs.docker.com

Let's learn Dockerfile.

So far, you learnt docker image vs docker container. Now lets deep dive into Dockerfile concept:

• A Dockerfile is a text file containing a set of instructions to build a Docker image. It's like a recipe that Docker follows to package your application and its dependencies into a portable container.

Let's break it down step by step:

Basic Structure of a Dockerfile

Here's an example Dockerfile for a Python web app running on Flask:

# Step 1: Base image
FROM python:3.9-slim

# Step 2: Set working directory
WORKDIR /app

# Step 3: Copy application code to the container
COPY . /app

# Step 4: Install dependencies
RUN pip install --no-cache-dir -r requirements.txt

# Step 5: Expose the application port
EXPOSE 5000

# Step 6: Define the command to run the application
CMD ["python", "app.py"]

Explanation of Each Line

1. Base Image:

   • Every Dockerfile starts with base image.
   • it can be an official image or custom image also.
   • The base image required a minimal environment needed your application.

2. Working directory:

   • Create the work directory inside your container.
   • All subsequent commands like COPY and RUN will operate relative to this directory.

3. Copy Application code to container:

   • Once you created work dir, you need to copy your code from host machine to container.
   • Syntax: COPY source destination.

4. Install dependencey:

   • Executes commands during the build process.
   • n this example, it installs Python dependencies from requirements.txt.

5. Expose the application port:

   • Tells Docker the container will listen on a specific port (e.g., 5000).
   • This is for documentation and doesn't actually open the port (you still need to map it with docker run -p).

6. Command to Run the App (CMD):

   • Specifies the default command to run when the container starts.
   • For example: python app.py launches a Flask application.

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Now we have instructions to build docker image, lets build our first image. I am going to use my another project for demo:

git clone https://github.com/infosecsingh/Weather-Check.git

Lets Run Build command.

1. Build the Image: Run the following in the directory containing your Dockerfile:

docker build -t weather-app:latest .

• build: used for building image with dockerfile instructions.
• -t: used for specify the tag, better for versioning.

Lets create container with image.

docker run -d -p 5000:5000 --name=weather-app weather-app

You can check running containers with below command

docker ps

Access your application with 5000 port on localhost

http://localhost:5000

Check Containers Stats.

docker stats 

Check container logs.

docker logs <container-id>

Lets push our first image.

before pushing to dockerhub or Artifact, tag your image properly.

docker tag <existing-image> <new-image-name>

For eg: run docker images

It is importment to give tag version, lets say you built 1.1version and then there are changes, then next time, you can build 1.2version.

docker tag 1nfosecsingh/app 1nfosecsingh/weather-app:v1

make sure 1nfosecsingh is mine username of docker account, you need to replace it.

Now, lets push to dockerhub, so that anyone can use our app.

docker push 1nfosecsingh/weather-app:v1

image-name:tag