{"id":17129,"date":"2017-05-19T12:15:29","date_gmt":"2017-05-19T09:15:29","guid":{"rendered":"https:\/\/www.webcodegeeks.com\/?p=17129"},"modified":"2017-05-19T11:54:50","modified_gmt":"2017-05-19T08:54:50","slug":"leveraging-dockerignore-file-create-smaller-images","status":"publish","type":"post","link":"https:\/\/www.webcodegeeks.com\/devops\/leveraging-dockerignore-file-create-smaller-images\/","title":{"rendered":"Leveraging the dockerignore File to Create Smaller Images"},"content":{"rendered":"

Keeping container image sizes small is one of the most common \u201cbest practice\u201d tips out there. There is good reason for this; it\u2019s very simple to let a container with a complex Dockerfile<\/code> and a large application turn into a large container image.<\/p>\n

A large container image can eventually become troublesome if left unchecked. When deploying a container into production, that production system must download the container from your registry. Ideally this process should be quick, however, network latency (i.e., downloading an image in London from a server in San Franscisco) can cause this process can take a long time.<\/p>\n

If you are using services to build your containers, a large container could easily cause those services to timeout. This is also true of deployment automation such as Puppet, SaltStack, or Ansible. Each of these services has a max execution time \u2014 a large image and a slow network connection could make for a messy or failed deployment.<\/p>\n

With that said, there are several techniques for keeping images small. In today\u2019s article, we will explore an often-ignored technique, using the .dockerignore<\/code> file.<\/p>\n

Exploring the .dockerignore File<\/h2>\n

The .dockerignore<\/code> file is a special file that can be placed within the build context directory<\/strong>. The build context directory is the directory that we specify at the end of a docker build<\/code> command. The file itself is a simple text file that contains a list of glob<\/a> patterns for files and directories to exclude from the final build image.<\/p>\n

By leveraging the .dockerignore<\/code> file, we can exclude files and directories we do not need within our final image. To explain this better, let\u2019s walk through a real world example.<\/p>\n

Adding .dockerignore<\/code> to an existing project<\/h3>\n

One of the most common locations to store a dockerfile<\/code> is the top level of the application\u2019s code repository. My personal projects are no exception. For this article, we will go ahead and add a .dockerignore<\/code> file to one of my personal open-source projects that leverages Docker.<\/p>\n

To get started, let\u2019s first clone the project.<\/p>\n

$ git clone https:\/\/github.com\/madflojo\/automatron.git<\/pre>\n
With the project cloned, let\u2019s take a look at the files included in this repository.<\/p>\n
$ cd automatron\/\r\n$ ls -la\r\ntotal 208\r\ndrwxr-xr-x  24 madflojo  users    816 Apr 15 23:00 .\r\ndrwxrwxrwt   7 root      users    238 Apr 15 23:00 ..\r\n-rw-r--r--   1 madflojo  users     29 Apr 15 23:00 .coveragerc\r\ndrwxr-xr-x  13 madflojo  users    442 Apr 15 23:00 .git\r\n-rw-r--r--   1 madflojo  users    834 Apr 15 23:00 .gitignore\r\n-rw-r--r--   1 madflojo  users   5760 Apr 15 23:00 CONTRIBUTING.md\r\n-rw-r--r--   1 madflojo  users    408 Apr 15 23:00 Dockerfile\r\n-rw-r--r--   1 madflojo  users  11343 Apr 15 23:00 LICENSE\r\n-rw-r--r--   1 madflojo  users    220 Apr 15 23:00 Procfile\r\n-rw-r--r--   1 madflojo  users   4538 Apr 15 23:00 README.md\r\n-rw-r--r--   1 madflojo  users   9411 Apr 15 23:00 actioning.py\r\ndrwxr-xr-x   4 madflojo  users    136 Apr 15 23:00 config\r\ndrwxr-xr-x   8 madflojo  users    272 Apr 15 23:00 core\r\n-rw-r--r--   1 madflojo  users   7842 Apr 15 23:00 discovery.py\r\n-rw-r--r--   1 madflojo  users    415 Apr 15 23:00 docker-compose.yml\r\ndrwxr-xr-x  10 madflojo  users    340 Apr 15 23:00 docs\r\n-rw-r--r--   1 madflojo  users   2208 Apr 15 23:00 mkdocs.yml\r\n-rw-r--r--   1 madflojo  users   8609 Apr 15 23:00 monitoring.py\r\ndrwxr-xr-x   9 madflojo  users    306 Apr 15 23:00 plugins\r\n-rw-r--r--   1 madflojo  users    114 Apr 15 23:00 requirements.txt\r\n-rw-r--r--   1 madflojo  users   5992 Apr 15 23:00 runbooks.py\r\ndrwxr-xr-x   5 madflojo  users    170 Apr 15 23:00 tests\r\n-rw-r--r--   1 madflojo  users   1018 Apr 15 23:00 tests.py<\/pre>\n
With just a quick look, we can see several files and directories that could be omitted from a production Docker image. Files and directories such as .git\/<\/code>, tests\/<\/code>, mkdocs.yml<\/code>, and even the CONTRIBUTING.md<\/code> file.<\/p>\n
Let\u2019s see if these files are included when we perform a docker build<\/code>.<\/p>\n
$ docker build -t automatron .<\/pre>\n
The Dockerfile<\/code> within this repository adds files using the following instruction.<\/p>\n
ADD . \/<\/pre>\n
This instruction essentially adds all of the files located within the build directory to the \/<\/code> directory within the container. We can see this if we run the container executing the ls -la<\/code> command.<\/p>\n
$ docker run automatron ls -la \/ | grep 2017\r\n-rw-r--r--   1 root root    29 Apr 16  2017 .coveragerc\r\ndrwxr-xr-x   8 root root  4096 Apr 16  2017 .git\r\n-rw-r--r--   1 root root   834 Apr 16  2017 .gitignore\r\n-rw-r--r--   1 root root  5760 Apr 16  2017 CONTRIBUTING.md\r\n-rw-r--r--   1 root root   408 Apr 16  2017 Dockerfile\r\n-rw-r--r--   1 root root 11343 Apr 16  2017 LICENSE\r\n-rw-r--r--   1 root root   220 Apr 16  2017 Procfile\r\n-rw-r--r--   1 root root  4538 Apr 16  2017 README.md\r\n-rw-r--r--   1 root root  9411 Apr 16  2017 actioning.py\r\ndrwxr-xr-x   3 root root  4096 Apr 16  2017 config\r\ndrwxr-xr-x   2 root root  4096 Apr 16  2017 core\r\n-rw-r--r--   1 root root  7842 Apr 16  2017 discovery.py\r\n-rw-r--r--   1 root root   415 Apr 16  2017 docker-compose.yml\r\ndrwxr-xr-x   6 root root  4096 Apr 16  2017 docs\r\n-rw-r--r--   1 root root  2208 Apr 16  2017 mkdocs.yml\r\n-rw-r--r--   1 root root  8609 Apr 16  2017 monitoring.py\r\n-rw-r--r--   1 root root   114 Apr 16  2017 requirements.txt\r\n-rw-r--r--   1 root root  5992 Apr 16  2017 runbooks.py\r\ndrwxr-xr-x   5 root root  4096 Apr 16  2017 tests\r\n-rw-r--r--   1 root root  1018 Apr 16  2017 tests.py<\/pre>\n
If we look above, we can see that all of the files from the build directory have been added to the container. Let\u2019s start excluding some of these files, starting with the .git\/<\/code> directory. I am starting with the .git\/<\/code> directory because it\u2019s a commonly large directory that can easily be overlooked.<\/p>\n
The .git\/<\/code> directory is a special directory that is used by git<\/code> to store all of the version control meta information. This includes details and even differences of each commit.<\/p>\n
This means the more active a project, the larger the .git\/<\/code> directory will be. For my project, the .git\/<\/code> directory is only 1MB in size. However, if we look at the Apache Cassandra project\u2019s .git\/<\/code> directory it is over 200MB<\/strong> in size. This is due to both the size of the codebase and the active nature of the project.<\/p>\n
For our example, the .git\/<\/code> directory might not add that much value, but if we were building a container from the Cassandra project\u2019s repository, removing the .git\/<\/code> directory would greatly reduce the size of the resulting container image.<\/p>\n
With that said, let\u2019s go ahead and add the .git\/<\/code> directory to a newly created .dockerignore<\/code> file. We can do this by adding the following:<\/p>\n
.git<\/code><\/p>\n
Once this line is added, let\u2019s build the container again and check the resulting contents.<\/p>\n
$ docker build -t automatron .\r\n$ docker run automatron ls -la \/ | grep 2017\r\n-rw-r--r--   1 root root    29 Apr 16  2017 .coveragerc\r\n-rw-r--r--   1 root root     5 Apr 16  2017 .dockerignore\r\n-rw-r--r--   1 root root   834 Apr 16  2017 .gitignore\r\n-rw-r--r--   1 root root  5760 Apr 16  2017 CONTRIBUTING.md\r\n-rw-r--r--   1 root root   408 Apr 16  2017 Dockerfile\r\n-rw-r--r--   1 root root 11343 Apr 16  2017 LICENSE\r\n-rw-r--r--   1 root root   220 Apr 16  2017 Procfile\r\n-rw-r--r--   1 root root  4538 Apr 16  2017 README.md\r\n-rw-r--r--   1 root root  9411 Apr 16  2017 actioning.py\r\ndrwxr-xr-x   3 root root  4096 Apr 16  2017 config\r\ndrwxr-xr-x   2 root root  4096 Apr 16  2017 core\r\n-rw-r--r--   1 root root  7842 Apr 16  2017 discovery.py\r\n-rw-r--r--   1 root root   415 Apr 16  2017 docker-compose.yml\r\ndrwxr-xr-x   6 root root  4096 Apr 16  2017 docs\r\n-rw-r--r--   1 root root  2208 Apr 16  2017 mkdocs.yml\r\n-rw-r--r--   1 root root  8609 Apr 16  2017 monitoring.py\r\n-rw-r--r--   1 root root   114 Apr 16  2017 requirements.txt\r\n-rw-r--r--   1 root root  5992 Apr 16  2017 runbooks.py\r\ndrwxr-xr-x   5 root root  4096 Apr 16  2017 tests\r\n-rw-r--r--   1 root root  1018 Apr 16  2017 tests.py<\/pre>\n
As we can see from the resulting output, the container is now missing the .git\/<\/code> directory.<\/p>\n
The above is a simple example of using the .dockerignore<\/code> file. At this point, we could simply add a similar entry for each file and directory we wish to omit and we could have a smaller resulting image. There is, however, an easier way.<\/p>\n
As I mentioned earlier, the .dockerignore<\/code> file understands Unix glob patterns. If, for example, we wanted to omit all files that started with a .<\/code>, we could simply add .*<\/code> to the file.<\/p>\n
Since the .dockerignore<\/code> file uses Unix style glob patterns, we can safely add .*<\/code> and only dot-files will be excluded.<\/p>\n
In addition to .*<\/code>, let\u2019s go ahead and add a few more items to omit.<\/p>\n
.*\r\ndocs\r\nmkdocs.yml\r\ndocker-compose.yml\r\ntest*\r\n*.md<\/pre>\n
In the above, we have some clearly specified items such as docs\/<\/code>, docker-compose.yml<\/code>, and mkdocs.yml<\/code>. We also have some glob patterns such as test*<\/code>, which will cause us to omit tests\/<\/code> and tests.py<\/code>. We also have another interesting one: *.md<\/code>, which will cause Docker to omit any markdown file such as README.md<\/code> and CONTRIBUTING.md<\/code>.<\/p>\n
Let\u2019s see how this comes together by running another build and ls -la<\/code>.<\/p>\n
$ docker build -t automatron .\r\n$ docker run automatron ls -la \/ | grep 2017\r\n-rw-r--r--   1 root root   408 Apr 16  2017 Dockerfile\r\n-rw-r--r--   1 root root 11343 Apr 16  2017 LICENSE\r\n-rw-r--r--   1 root root   220 Apr 16  2017 Procfile\r\n-rw-r--r--   1 root root  9411 Apr 16  2017 actioning.py\r\ndrwxr-xr-x   3 root root  4096 Apr 16  2017 config\r\ndrwxr-xr-x   2 root root  4096 Apr 16  2017 core\r\n-rw-r--r--   1 root root  7842 Apr 16  2017 discovery.py\r\n-rw-r--r--   1 root root  8609 Apr 16  2017 monitoring.py\r\n-rw-r--r--   1 root root   114 Apr 16  2017 requirements.txt\r\n-rw-r--r--   1 root root  5992 Apr 16  2017 runbooks.py<\/pre>\n
The output this time is quite a bit less than our previous run. We can see that we are now missing the files we wanted to omit.<\/p>\n
At this point, we have achieved our goal: we eliminated files that were not needed within our final image. There is, however, one file missing that I wanted to include.<\/p>\n
!Sign up for a free Codeship Account<\/p>\n
Using ! to include files<\/h3>\n
The missing file is the README.md<\/code> file. In the .dockerignore<\/code> file, we added a line *.md<\/code> to omit all markdown files. My project has a few markdown files already and I fully expect more to pop up in the future.<\/p>\n
The problem is I\u2019d like to include only the README.md<\/code> and no other markdown files. I\u2019d also like to not have to specify each and every markdown file to accomplish this. Luckily, Docker provides this ability.<\/p>\n
By adding the following, we can keep our removal of all markdown files but still retain our README.md<\/code>:<\/p>\n
.*\r\ndocs\r\nmkdocs.yml\r\ndocker-compose.yml\r\ntest*\r\n*.md\r\n!README.md<\/pre>\n
With the above, we simply added the README.md<\/code> file with the !<\/code> character in front of it. This tells Docker to include the README.md<\/code> or rather exclude it from other exclusions.<\/p>\n
Let\u2019s go ahead and see what files our container includes with our changes applied.<\/p>\n
$ docker build -t automatron .\r\n$ docker run automatron ls -la \/ | grep 2017\r\n-rw-r--r--   1 root root   408 Apr 16  2017 Dockerfile\r\n-rw-r--r--   1 root root 11343 Apr 16  2017 LICENSE\r\n-rw-r--r--   1 root root   220 Apr 16  2017 Procfile\r\n-rw-r--r--   1 root root  4538 Apr 16  2017 README.md\r\n-rw-r--r--   1 root root  9411 Apr 16  2017 actioning.py\r\ndrwxr-xr-x   3 root root  4096 Apr 16  2017 config\r\ndrwxr-xr-x   2 root root  4096 Apr 16  2017 core\r\n-rw-r--r--   1 root root  7842 Apr 16  2017 discovery.py\r\n-rw-r--r--   1 root root  8609 Apr 16  2017 monitoring.py\r\n-rw-r--r--   1 root root   114 Apr 16  2017 requirements.txt\r\n-rw-r--r--   1 root root  5992 Apr 16  2017 runbooks.py<\/pre>\n
With the !README.md<\/code> entry added, we can now see our README.md<\/code> was included but not our CONTRIBUTING.md<\/code>. This means our instruction to omit all markdown files (*.md<\/code>) was applied to all except the README.md<\/code>.<\/p>\n
Summary<\/h2>\n
In this article, we covered how to leverage the .dockerignore<\/code> file to exclude unnecessary files and directories from the container build. As we found out, the usage of the .dockerignore<\/code> file is very simple. Do you have any .dockerignore<\/code> tips or tricks? Add it to the comments or tweet it to us.<\/p>\n
\n\n\n\n
Reference: <\/span><\/td>\nLeveraging the dockerignore File to Create Smaller Images<\/a> from our WCG partner<\/a>\u00a0Ben Cane at the Codeship Blog<\/a> blog.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
Keeping container image sizes small is one of the most common \u201cbest practice\u201d tips out there. There is good reason for this; it\u2019s very simple to let a container with a complex Dockerfile and a large application turn into a large container image. A large container image can eventually become troublesome if left unchecked. When …<\/p>\n","protected":false},"author":158,"featured_media":10356,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[217],"class_list":["post-17129","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-devops","tag-docker"],"yoast_head":"\nLeveraging the dockerignore File to Create Smaller Images - Web Code Geeks - 2026<\/title>\n<meta name=\"description\" content=\"Keeping container image sizes small is one of the most common \u201cbest practice\u201d tips out there. 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It is important to note that Unix style glob patterns<\/a> are not regular expressions. .*<\/code> is a prime example of this. In a \u201cglob\u201d pattern, this matches everything that starts with a .<\/code>. In a regular expression, this would match every character, essentially matching every file and directory.<\/p>\n