1. Before We Dive In: What Does “Working of a Docker Image” Mean?
When someone says:
“Bro, how does a Docker image work internally?”
They really mean:
- How Docker builds an image
- How layers are created
- How Docker stores these layers
- How images turn into containers
- How Docker decides what to rebuild
- How caching makes builds fast
So in this article, we’ll break the full process visually and simply.
2. Visual Understanding: How Docker Handles an Image
Here’s the high-level visual:
Dockerfile
↓ docker build
Layers created
↓
Final Docker Image
↓ docker run
Container (running app)
So the image is the middle step between:
- Instructions (Dockerfile)
- Running application (container)
3. How Docker Builds an Image (The Visual Step-by-Step)
Let’s assume this simple Dockerfile:
FROM node:18
WORKDIR /app
COPY . .
RUN npm install
CMD ["node", "app.js"]
Docker processes each line like a checkpoint.
✔ Step 1 — Start with Base Layer
FROM node:18
Visual:
Layer 1: Full Node 18 base image
This includes:
- Alpine OS
- Node runtime
- System tools
✔ Step 2 — Create Working Directory
WORKDIR /app
Docker creates a new layer for workspace setup.
Layer 2: /app folder
✔ Step 3 — Copy Application Files
COPY . .
Docker adds your code as a new read-only layer.
Layer 3: Your project files
✔ Step 4 — Install Dependencies
RUN npm install
Docker runs npm install and stores the result as another layer.
Layer 4: node_modules + installed packages
✔ Step 5 — Startup Instruction
CMD ["node", "app.js"]
This doesn’t create a big layer. It only sets the default start command.
Layer 5: Run instructions (metadata)
So the final image is a stack of layers:
Layer 5: CMD
Layer 4: Installed packages
Layer 3: Your code
Layer 2: Workdir setup
Layer 1: Base OS + Node
🔍 4. Why Docker Uses Layers (Very Important)
Because layers make Docker:
✔ Fast
Only changed layers rebuild.
✔ Efficient
Shared layers = smaller downloads.
✔ Cache-friendly
If nothing changed in a line, Docker reuses the old layer.
✔ Portable
Layered structure makes images easy to move across machines.
Visual caching example:
COPY package.json → unchanged
RUN npm install → cached
COPY . . → changed
Only the last layer rebuilds → everything else is reused.
5. Where Are Docker Image Layers Stored?
On your machine:
/var/lib/docker/
Docker stores each layer as a content-addressed blob.
You don't manage this — Docker handles it behind the scenes.
6. How Docker Converts an Image Into a Running Container
When you run:
docker run my-app
Docker does:
✔ Step 1 → Load layers
L1
L2
L3
L4
L5
✔ Step 2 → Create a thin writable layer on top
(So your container can modify data)
Visual:
Writable Container Layer
-------------------------
Image Layers (read-only)
✔ Step 3 → Execute CMD
Starts your app exactly as defined in image.
This is how your app becomes alive.
7. Visual Summary (5-Second Memory Trick)
Dockerfile → Instructions
docker build → Layers created
Image → Stack of read-only layers
docker run → Adds writable top layer
Container → Running instance of image
8. Real Example: Inspect How an Image Actually Looks
Run:
docker image inspect node:18
You will see details like:
- Size
- Layers
- Metadata
- Commands used to build it
Run:
docker history node:18
This shows every layer and its size.
This is the best way to SEE how images work inside.
**9. FAQs **
Q1. Is a Docker Image the same as a container?
No. Image = blueprint. Container = running instance.
Q2. Can I edit a Docker Image?
No, it is read-only. You must rebuild.
Q3. Why do Docker Images have layers?
To speed up builds and reuse unchanged parts.
Q4. How does Docker know which layers to rebuild?
It checks whether the instruction or input files changed.
Q5. Can multiple containers run from the same image?
Yes — unlimited containers can be created.
🏁 Final Words (In Your Tone)
A Docker Image is basically your app packed into layers like a well-organized sandwich. Each line in your Dockerfile adds one more layer. When you run the image, Docker just puts a tiny writable layer on top and starts your app.
Once you understand how images actually work, Docker stops feeling complex — because everything becomes a predictable flow:
“Instructions → Layers → Image → Container → Running app.”
That’s the real magic of Docker by Example.