How do you add items to .dockerignore?

Question

I'm not able to find many examples of what a .dockerignore file should look like.

Using puppet to install a few packages on a docker container causes the image to explode from 600MB to 3GB. I'm trying to use a .dockerignore file to keep the size to a minumum

$ cat Dockerfile  
FROM centos:centos6

#Work around selinux problem on cent images
RUN yum install -y --enablerepo=centosplus libselinux-devel

RUN yum install -y wget git tar openssh-server; yum -y clean all

Add Puppetfile / 
RUN librarian-puppet install
RUN puppet apply --modulepath=/modules -e "class { 'buildslave': jenkins_slave => true,}"
RUN librarian-puppet clean

If I run docker images --tree I can see that the image instantlly grows by several GB

$ docker images --tree
                ├─e289570b5555 Virtual Size: 387.7 MB
                │ └─a7646acf90d0 Virtual Size: 442.5 MB
                │   └─d7bc6e1fbe43 Virtual Size: 442.5 MB
                │     └─772e6b204e3b Virtual Size: 627.5 MB
                │       └─599a7b5226f4 Virtual Size: 627.5 MB
                │         └─9fbffccda8bd Virtual Size: 2.943 GB
                │           └─ee46af013f6b Virtual Size: 2.943 GB
                │             └─3e4fe065fd07 Virtual Size: 2.943 GB
                │               └─de9ec3eba39e Virtual Size: 2.943 GB
                │                 └─31cba2716a12 Virtual Size: 2.943 GB
                │                   └─52cbc742d3c4 Virtual Size: 2.943 GB
                │                     └─9a857380258c Virtual Size: 2.943 GB
                │                       └─c6d87a343807 Virtual Size: 2.964 GB
                │                         └─f664124e0080 Virtual Size: 2.964 GB
                │                           └─e6cc212038b9 Virtual Size: 2.964 GB Tags: foo/jenkins-centos6-buildslave:latest

I believe the reason that the image grows so large, is because librarian-puppet clones a puppet module to /modules which breaks the build cache

I've tried the following .dockerignore files with no luck.

$ cat .dockerignore
/modules
/modules/
/modules/*

Is this the correct syntax for a .dockerignore file?
Are there any other ways to prevent these containers from growing so large?

Additional information:

http://kartar.net/2013/12/building-puppet-apps-inside-docker/
http://danielmartins.ninja/posts/a-week-of-docker.html

Answer 1

.dockerignore is to prevent files from being added to the initial build context that is sent to the docker daemon when you do docker build, it doesn't create a global rule for excluding files from being created in all images generated by a Dockerfile.

It's important to note that each RUN statement will generate a new image, with the parent of that image being the image generated by the Dockerfile statement above it. Try collapsing your RUN statements into a single one to reduce image size:

RUN librarian-puppet install &&\
 puppet apply --modulepath=/modules -e "class { 'buildslave': jenkins_slave => true,}" &&\
 librarian-puppet clean