FROM instruction for base image in Docker - Step-by-Step Execution

Process Flow - FROM instruction for base image

Start Dockerfile

↓

Read FROM instruction

↓

Pull base image if not local

↓

Set base image as build stage

↓

Continue with next instructions

↓

Build image on top of base

Docker reads the FROM instruction first, pulls the base image if needed, and sets it as the starting point for the build.

Execution Sample

Docker

FROM ubuntu:22.04
RUN apt-get update
RUN apt-get install -y curl

This Dockerfile starts from the Ubuntu 22.04 base image, updates package lists, and installs curl.

Process Table

Step	Instruction	Action	Result
1	FROM ubuntu:22.04	Check local images; pull if missing	Base image ubuntu:22.04 set
2	RUN apt-get update	Run update command inside base image	Package lists updated
3	RUN apt-get install -y curl	Install curl package	curl installed
4	End of Dockerfile	Build complete	Final image created

💡 All instructions executed; image built on ubuntu:22.04 base

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Base Image	None	ubuntu:22.04	ubuntu:22.04	ubuntu:22.04	ubuntu:22.04 with curl
Packages Installed	None	None	None	curl	curl
Package Lists	Outdated	Updated	Updated	Updated	Updated

Key Moments - 2 Insights

Why does Docker pull the base image during the FROM instruction?

Docker pulls the base image if it is not already on the local machine to ensure the build starts from the correct environment, as shown in execution_table step 1.

Can you have multiple FROM instructions in one Dockerfile?

Yes, multiple FROM instructions create multiple build stages, but each FROM sets a new base image, starting a new stage. This example shows a single FROM for simplicity.

Visual Quiz - 3 Questions

Look at the execution table, what is the base image set after step 1?

A None

B curl

C ubuntu:22.04

D apt-get

💡 Hint

Check the 'Result' column in step 1 of the execution_table.

At which step are package lists updated?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Result' columns in the execution_table for step 2.

If the base image was already local, what would change in step 1?

A Docker would skip pulling and use local image

B Docker would pull the image anyway

C Docker would fail the build

D Docker would remove the local image

💡 Hint

Refer to the 'Action' description in step 1 about checking local images.

Concept Snapshot

FROM instruction sets the base image for your Docker build.
Docker pulls this image if not present locally.
All following instructions build on top of this base.
You can have multiple FROMs for multi-stage builds.
Always specify a valid image name and tag.

Full Transcript

The FROM instruction in a Dockerfile tells Docker which base image to use for building your container image. When Docker reads FROM, it checks if the image is available locally. If not, it downloads (pulls) the image from a registry. This base image becomes the starting point for all following build steps. For example, FROM ubuntu:22.04 sets the base to Ubuntu 22.04. Then commands like RUN apt-get update run inside that base. The build ends when all instructions are done, producing a new image layered on top of the base. Multiple FROM instructions can create multi-stage builds, each starting fresh with a new base image.