Docker Introduction Basics

Docker

Docker is a tool that lets you package software into an image, and then run that image in an isolated environment called a container.

So why is this useful

• A docker images includes all the files you need to run an application
• All the OS system files, C libraries, the correct versions python for example
• The Docker images know how to start the application
• And on top of that the application will run the exact same way

How Docker Runs

Docker is a client/server application, the command line tool docker sends commands to a server, dockered

I can use a Docker Image to run a container

	# the docker run command is used to run a docker container
	docker run httpd:latest
	

Running a container interactively

docker run --entrypoint=/bib/sh -it httpd:alpinr

The --entrypoint argument means run a different starting command than the default command What this argument allows is to have a shell prompt inside the running container.

Docker containers have isolated file systems, isolated processes, and isolated users

Where does the container file system come from?

When you run a program on a computer you start with an executable files on the file-system, on disk, and use that file to create a process which is basically a running program. Multiple processes can be run from the same executable file.

One can think of a Docker as an executable file on disk, but instead one file, it is a whole file system, with everything needed to run the container. All the libraries, executable, and other files in a Linux file system to run the executable in a self container isolated execution .

A single image can create multiple containers, and each time a container is created, it gets a new complete and separate copy of the image file-system

Minimal Docker Packaging

For this example I will create a program that i want to run

mkdir docker-image
cd docker-image
touch hello.py
 

	# the contents of the hello.py file
from cosway import ghostbusters
ghostbusters("Who You Gonna Call")
 

python hello.py

To end the suspense this program does not run

Traceback (most recent call last):
  File "/home/laborant/docker-image/hello.py", line 1, in <module>
    from cosway import ghostbusters
ModuleNotFoundError: No module named 'cosway'
	
 
 

It fails because I need the library cosway, and this library is not part of the standard Python library.

This library needs to be installed in the Docker image. As stated earlier, a Docker image contains a complete file-system, In order to run hello.py, I will need

• A Python Interpreter
• The cosway library
• The file hello.py

The Docker image also includes instructions on what to do when the container/ application is run.

The Base Image

to create an image what is needed is a files called a Dockerfile. The Dockerfile is akin to a recipe used by the docker build command

Lets start with a very simple Dockerfile

FROM python:3.11-slim
 
 

This command simply means build the Dockerfile in the current directory. The . means current directory

 
# to build a docker image simply
 
docker image build .

To label a docker image with a name

 
docker image build -t hello .

To See a list of docker images

docker image list