Build context concept in Docker - Step-by-Step Execution

Process Flow - Build context concept

User runs docker build

↓

Docker CLI sends build context folder

↓

Docker daemon receives context

↓

Docker daemon reads Dockerfile

↓

Docker daemon executes instructions using context files

↓

Image layers created

↓

Build completes, image ready

This flow shows how Docker sends the build context folder to the daemon, which uses it to build the image step-by-step.

Execution Sample

Docker

docker build -t myapp .

This command builds a Docker image named 'myapp' using the current folder as the build context.

Process Table

Step	Action	Details	Result
1	Run docker build	Command: docker build -t myapp .	Starts build, context is current folder
2	Send build context	Docker CLI sends all files in '.' folder	Context files transferred to daemon
3	Read Dockerfile	Docker daemon reads Dockerfile in context	Instructions loaded
4	Execute instructions	Uses context files for COPY, ADD commands	Image layers created
5	Complete build	All steps done	Image 'myapp' created locally

💡 Build finishes after all Dockerfile instructions run using the provided context files

Status Tracker

Variable	Start	After Step 2	After Step 4	Final
build_context	empty	current folder files sent	files available for Dockerfile commands	used fully for image build
image_layers	none	none	layers created per instruction	final image layers ready

Key Moments - 2 Insights

Why does Docker send the whole folder as context even if only one file is needed?

Docker sends the entire folder specified as context (here '.') because it cannot know in advance which files the Dockerfile will use. See execution_table step 2 where all files are sent.

What happens if a needed file is outside the build context folder?

Docker cannot access files outside the build context. This causes build errors during execution (step 4). The context must include all needed files.

Visual Quiz - 3 Questions

Look at the execution table, what is sent to the Docker daemon at step 2?

A All files in the current folder (build context)

B Only the Dockerfile

C No files, just instructions

D Only files listed in Dockerfile

💡 Hint

Check the 'Details' column in step 2 of the execution_table

At which step does Docker create image layers?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Result' column in step 4 of the execution_table

If the build context folder is changed to a subfolder, what changes in the execution?

A Docker sends files from the parent folder

B Docker sends files from the subfolder as context

C Docker ignores the context and uses current folder

D Docker sends no files

💡 Hint

Recall that the context folder is what Docker CLI sends as per step 2

Concept Snapshot

Docker build context is the folder sent to the daemon during build.
It includes all files in that folder and subfolders.
Dockerfile instructions like COPY use files from this context.
If files are outside context, build fails.
Use '.' to send current folder as context.
Context size affects build speed.

Full Transcript

When you run 'docker build -t myapp .', Docker sends the current folder as the build context to the Docker daemon. The daemon reads the Dockerfile inside this context and executes each instruction. Files in the context folder are available for commands like COPY or ADD. Docker creates image layers step-by-step using these files. The build finishes when all instructions run successfully, producing the image named 'myapp'. If needed files are outside the context folder, the build will fail because Docker cannot access them. The build context is important because it defines what files Docker can use during the build.