How to ignore files in Docker?

Introduction

Here in this article, we will learn about the ignore file in docker. How to create a ignore file and how to use it with various ways to add files and folders to the ignore file. All the pre-requisite knowledge is mentioned in a stepwise manner. Do not directly jump to the implementation without knowing the requirement of the ignore files in the code industry.

What are ignored files?

Ignore files created in different systems like in the Git repository and the Dockerifle. These files help to exclude some files and directories from being used or copied in the output. The output for the dockerfile is the image and for the git, it could be the commit. Examples of some most commonly used ignore files in DevOps are “gitignore” and “dockerignore”.

Types of files to be ignored

The main task is to ignore things. These things can be of different types. Like it could be a program file in python or a ReactJS app directory with sub-directories. All these files are mentioned in the below list.

• Directory

• File − text or program files

• Sub−directory

• .dockerignore itself

The need of ignoring these files is because we no longer need these files in our final product. These files only need on the Host.

Method

In this section, we will learn about excluding different types of files by adding them to the dockerignore file.

All the files that are going to be in the dockerignore should be mentioned with respect to the docker context directory. Docker context is that directory path where all the useful files that need to be added inside the image are present. Mostly we use . (dot) as the docker context while building the image.

Create a “.dockerignore” file

The “.dockerignore” should be created where your Dockerfile is present otherwise it won’t work. Use the below commands to create and exclude various types of files.

$touch .dockerignore

We have created a directory structure to use in explaining all these methods.

$tree -la

Output

.
├── dir1
├── dir2
├── dir3
│ ├── dir3_1
│ │ └── file_dir3_1.txt
│ ├── dir3_2
│ ├── file1_dir3.txt
│ └── file2_dir3.py
├── Dockerfile
├── .dockerignore
├── file1.txt
├── file2.sh
├── file3.py
├── no_ex1
└── No_ex2


5 directories, 10 files

Open the ignore file to add various files from the directory structure above.

$nano .dockerignore

There are some rules to add different kinds of files, directories, and comments in the docker ignore file.

Add a comment in the dockerignore file

Using a # while starting a comment will consider that line as a comment.

#This is a comment in the .dockerignore file

Ignore a directory at the level of the docker context or root directory

dir1
dir2

This will ignore the directories (dir1 & dir2) that are present at the root level of the docker context. For reference see the above directory structure.

Ignore all the files and directory

To exclude all the files and directories that have “file” in their name. These files will be from the root directory only.

file*

Ignore sub-directory up to one level down of the docker context

Ignore the files as well as the directories from any sub-directories of the root directory.

#using */ to go one level down
*/file1_dir3.txt

Ignore a sub-directory up to the two-level down of the docker context

This is very similar to the above method.

#using */*/ to go two level down
*/*/file_dir3_1.txt

To reinclude a file

There are some special cases when we want to include a particular file that was excluded earlier.

*.txt
!myfile.txt

Suppose you are creating a python container image. The root directory containers all the python programs from different contributors so you can’t delete them and you have to use that root directory only. You could perform the below example to exclude all the python programs and then reinclude your python program file.

Step 1 − suppose the root directory is filled with pythons files

Prg1.py. Prg2.py, Prg3.py …………..and myprogram.py

Step 2 − Create a .dockerignore file as below.

#ignore all the python program files
*.py
#reinclude your python program file
!myprogram.py

The process will be faster and no need to COPY/ADD unwanted things to the docker container image.

Implementation

Here we are going to create a complete environment that will use the Dockerfile, .dockerignore, and build a container image using these files. The same directory structure is used as mentioned in the method section.

.
├── dir1
├── dir2
├── dir3
│ ├── dir3_1
│ │ └── file_dir3_1.txt
│ ├── dir3_2
│ ├── file1_dir3.txt
│ └── file2_dir3.py
├── Dockerfile
├── .dockerignore
├── file1.txt
├── file2.sh
├── file3.py
├── no_ex1
└── No_ex2

5 directories, 10 files

Step 1 − Create a Dockerfile and add the code.

$nano Dockerfile

Input 

#for the base image we are using busybox
FROM busybox:latest
#set a working directory
WORKDIR /my_context_directory
#copy everything from the build context to the working directory of the
#image
COPY . .
# list all the copied items
RUN ls -l

Save the file.

Step 2 − Create a .dockerignore file

$nano .dockerignore

Input 

#exclude the file_dir3_1.txt
file_dir3_1.txt
#exclude file from the root
file3.py
#exclude a directory one level down
*/dir3_2

Save the file

Step 3 − Build the image and check the build output

$docker build –t test_ignore .

Output

Sending build context to Docker daemon 9.728kB
Step 1/4 : FROM busybox:latest
   ---> 9d5226e6ce3f
Step 2/4 : WORKDIR /my_context_directory
   ---> Running in 49d65f57621d
Removing intermediate container 49d65f57621d
   ---> 386c30f8f511
Step 3/4 : COPY . .
   ---> 75f65dace2be
Step 4/4 : RUN ls -l
   ---> Running in bd8bbec7d3f9
total 12
-rw-rw-r-- 1 root root 0 Dec 9 03:47 No_ex2
drwxrwxr-x 2 root root 4096 Dec 9 03:47 dir1
drwxrwxr-x 2 root root 4096 Dec 9 03:47 dir2
drwxrwxr-x 3 root root 4096 Dec 9 03:48 dir3
-rw-rw-r-- 1 root root 0 Dec 9 03:46 file1.txt
-rw-rw-r-- 1 root root 0 Dec 9 03:46 file2.sh
-rw-rw-r-- 1 root root 0 Dec 9 03:47 no_ex1
Removing intermediate container bd8bbec7d3f9
   ---> 084bb699260f
Successfully built 084bb699260f
Successfully tagged test_ignore:latest

The output shows that all the files other than the files in the dockerignore files have been copied to the container image.

Conclusions

Here in this article, we have explored a complete 360-degree view of the “.dockerignore” file. Excluding, including, and the importance of this file is clearly stated. To get hands-on with the topic create different environments and implement with above-mentioned methods.