Docker Part 2- Image Management

 

                                                                  Docker Image Management 

A docker image is a read only blue print used to create containers. It contains the application code, dependencies and environment required to run the containers.

                                             Image vs Container 

Feature	Docker Image	Docker Container
Definition	Read-only template used to create containers	Running instance of a Docker image
State	Static (does not change)	Dynamic (changes during runtime)
Purpose	Used to build containers	Used to run applications
Mutability	Immutable (cannot be modified)	Mutable (can be modified)
Storage	Stored on disk	Runs in memory + disk
Creation	Created using Dockerfile	Created using docker run
Lifecycle	Exists until deleted	Starts, stops, restarts
Reusability	Can be reused multiple times	Usually short-lived
Read/Write Layer	No write layer	Has a writable layer
Example	nginx:latest	Running nginx server

 

                                                               Commands 

1. docker images --> list docker image 

2. docker pull image name --> pull (download) image 

3. docker build -t image name --> Build docker image 

4. docker rmi image id --> remove docker image 

5. docker rmi -f  image id --> remove the docker image in the system 

6. docker tag httpd httpd:v1 --> tag docker image 

7. docker inspect image name --> show all metadata image 

8. docker history image name --> show image history 

9. docker save -o myimage.tar image name --> save the image in .tar file

Types of Image Creation

1. DockerFile Method 

2. Commit Method

Docker file :- A docker file is a text file that contains a set of  instruction used to build a Docker Image.

It defines everything your image should have - like base image, software package, environment variables, coping files and command to run.

When you run "docker build", docker reads ths docker file line-by-line and creates a custom, repeatable and automated image.

🧩 Why Dockerfile is used?

• To create custom Docker images

• To make application deployment repeatable

• To automate image creation

• To follow DevOps best practices

📄 Very Small Example Dockerfile (Beginner Friendly)

Example: Simple “Hello Docker” Image

FROM ubuntu

CMD ["echo", "Hello Docker"]

🔍 Explanation:

• FROM ubuntu → Base image

• CMD → Command that runs when container starts

▶️ Build & Run

docker build -t hello-docker .
docker run hello-docker

Output:
Hello Docker

📝 One-Line Summary (Blog Highlight)


A Dockerfile is a script that defines how a Docker image is built and how the container runs.

🐳 Dockerfile Important Instructions (Complete Table)

Instruction	Purpose	Runs At	Example
FROM	Base image define karta hai	Build time	FROM ubuntu:22.04
RUN	Commands execute karta hai (install, update)	Build time	RUN apt-get update
COPY	Local files image me copy karta hai	Build time	COPY app.js /app/
ADD	COPY + extra features (URL, extract)	Build time	ADD app.tar.gz /app/
CMD	Container start hone par default command	Run time	CMD ["node","app.js"]
ENTRYPOINT	Fixed command set karta hai	Run time	ENTRYPOINT ["nginx"]
EXPOSE	Container port document karta hai	Build time	EXPOSE 80
ENV	Environment variables set karta hai	Build time / Run time	ENV APP_ENV=prod
WORKDIR	Working directory set karta hai	Build time	WORKDIR /app
USER	Container kis user se run hoga	Run time	USER node
VOLUME	Persistent data ke liye volume	Run time	VOLUME /data
LABEL	Image metadata add karta hai	Build time	LABEL author="DevOps"
ARG	Build-time variables define karta hai	Build time	ARG VERSION=1.0
HEALTHCHECK	Container health check karta hai	Run time	HEALTHCHECK CMD curl localhost
SHELL	Default shell change karta hai	Build time	SHELL ["/bin/bash","-c"]
ONBUILD	Child image ke liye trigger	Build time	ONBUILD COPY . /app
STOPSIGNAL	Container stop signal define karta hai	Run time	STOPSIGNAL SIGTERM

Difference between Copy and ADD in Dockerfile?

Copy and ADD both copy files from the host to the container. The difference is, Copy is simple and only copies local files or directories, while ADD can also download files from a URL and automatically extract .tar archives.

Best practice: always use COPY for simple coping and use ADD only when extra feature are needed.

Difference between CMD and ENTRYPOINT in Dockerfile?

ENTRYPOINT sets the main command , CMD sets default arguments. CMD is easily overridden at runtime, ENTRYPOINT is not, use ENTRYPOINT for fixed commands and CMD for default.