What is Docker - Visual Explanation

Process Flow - What is Docker

Write Dockerfile

↓

Build Docker Image

↓

Run Docker Container

↓

App runs isolated

↓

Stop and Remove Container

Docker lets you package an app and its environment into a container that runs the same everywhere.

Execution Sample

Docker

FROM alpine:latest
RUN echo "Hello from Docker!"
CMD ["echo", "Hello from Docker!"]

This Dockerfile creates a small image that prints a message when run.

Process Table

Step	Action	Result	Notes
1	Read Dockerfile	Dockerfile parsed	Start building image
2	Pull base image alpine:latest	Base image downloaded	Alpine Linux is small and fast
3	Run command: echo "Hello from Docker!"	Command executed in image build	Creates a layer with this command
4	Set default command CMD ["echo", "Hello from Docker!"]	Default container command set	Runs when container starts
5	Build image complete	Image created with layers	Ready to run container
6	Run container from image	Container starts and runs echo	Outputs: Hello from Docker!
7	Container stops after command	Container exits	No running process left
8	Remove container	Container deleted	Clean up resources

💡 Container stops because the echo command finishes, no ongoing process

Status Tracker

Variable	Start	After Step 2	After Step 5	After Step 6	Final
Image	None	alpine:latest pulled	Image built with echo command	Image used to run container	Image remains on system
Container	None	None	None	Running with echo command	Stopped and removed

Key Moments - 3 Insights

Why does the container stop immediately after running?

Because the container runs the echo command which finishes quickly, so the container exits as shown in execution_table step 7.

What is the difference between an image and a container?

An image is a packaged app with its environment (execution_table step 5), while a container is a running instance of that image (step 6).

Why do we need a base image like alpine?

The base image provides a minimal operating system environment for the app to run, as seen in step 2 where alpine is pulled.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A The echo command runs during image build

B The container starts running

C The base image is downloaded

D The container is removed

💡 Hint

Check the 'Action' and 'Result' columns at step 3 in the execution_table

At which step does the container stop running?

A Step 5

B Step 6

C Step 7

D Step 8

💡 Hint

Look for 'Container exits' in the 'Result' column in execution_table

If we change CMD to run a long process, how would step 7 change?

A Container would stop immediately

B Container would keep running longer

C Image would fail to build

D Container would be removed automatically

💡 Hint

Step 7 shows container stops because echo finishes; a long process would delay this

Concept Snapshot

Docker packages apps with their environment into images.
Images are built from Dockerfiles using base images.
Containers are running instances of images.
Containers run isolated and can be started, stopped, and removed.
Docker ensures apps run the same everywhere.

Full Transcript

Docker is a tool that packages an application and its environment into a container. This container runs the same on any computer. The process starts by writing a Dockerfile that describes the app and its environment. Then, Docker builds an image from this file, starting with a base image like alpine Linux. The image includes commands to run the app. When you run the image, Docker creates a container that runs the app isolated from the rest of the system. Once the app finishes, the container stops and can be removed to free resources. This makes it easy to share and run apps consistently.