Using .dockerignore - Step-by-Step Execution

Process Flow - Using .dockerignore

Start Docker Build

↓

Read .dockerignore file

↓

Check each file/folder in context

↓

If matches .dockerignore pattern?

Yes→Exclude from build context

No↓

Include in build context

↓

Send build context to Docker daemon

↓

Build image with included files

↓

Finish

Docker reads the .dockerignore file to exclude specified files/folders from the build context before building the image.

Execution Sample

Docker

FROM alpine
COPY . /app
RUN ls /app

Builds an image copying current folder except files ignored by .dockerignore, then lists copied files.

Process Table

Step	File/Folder Checked	Matches .dockerignore?	Action	Build Context State
1	node_modules/	Yes	Exclude	All files except node_modules/
2	README.md	No	Include	README.md included
3	src/	No	Include	src/ included
4	.git/	Yes	Exclude	node_modules/, .git/ excluded
5	Dockerfile	No	Include	Dockerfile included
6	temp.log	Yes	Exclude	node_modules/, .git/, temp.log excluded
7	All files checked	-	-	Final context excludes node_modules/, .git/, temp.log

💡 All files checked; excluded files are not sent to Docker daemon for build.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
Build Context	All files	Exclude node_modules/	Include README.md	Include src/	Exclude .git/	Include Dockerfile	Exclude temp.log	Final context excludes node_modules/, .git/, temp.log

Key Moments - 2 Insights

Why are some files not copied into the Docker image even though they exist in the folder?

Because those files match patterns in .dockerignore and are excluded from the build context, as shown in execution_table steps 1, 4, and 6.

Does .dockerignore affect files copied by COPY commands inside the Dockerfile?

Yes, because COPY only copies files present in the build context. Files excluded by .dockerignore are not in the context and thus not copied, as seen in the final build context state.

Visual Quiz - 3 Questions

Look at the execution table, which file/folder is excluded at step 4?

A README.md

B src/

C .git/

D Dockerfile

💡 Hint

Check the 'File/Folder Checked' and 'Matches .dockerignore?' columns at step 4.

At which step is 'temp.log' excluded from the build context?

A Step 6

B Step 3

C Step 2

D Step 5

💡 Hint

Look for 'temp.log' in the 'File/Folder Checked' column and see the action taken.

If you remove 'node_modules/' from .dockerignore, what changes in the build context?

A node_modules/ will still be excluded

B node_modules/ will be included in the build context

C All files will be excluded

D Only Dockerfile will be included

💡 Hint

Refer to the variable_tracker showing exclusion of node_modules/ and understand .dockerignore effect.

Concept Snapshot

Using .dockerignore:
- Place .dockerignore in build context root
- List files/folders to exclude from build context
- Docker skips these during build context upload
- Speeds up build and reduces image size
- Patterns support wildcards and negation

Full Transcript

When Docker builds an image, it sends all files in the build folder to the Docker daemon. The .dockerignore file tells Docker which files or folders to skip. This helps avoid sending unnecessary files like node_modules or .git. Docker reads .dockerignore first, then checks each file. If a file matches a pattern, it is excluded from the build context. Only included files are sent and can be copied into the image. This makes builds faster and images smaller. For example, if node_modules/ is in .dockerignore, it won't be copied into the image even if the Dockerfile says COPY . /app. The execution table shows each file checked and whether it is included or excluded. The variable tracker shows how the build context changes step by step. Understanding this helps avoid accidentally copying large or sensitive files into images.