Background
When you deploy an application, our build system is fetching all the required dependencies according to the programming language and the requirements of your project. For compiled languages, their executable or package is then built. All these files are gathered in an application image. This image is stored and distributed to our servers when they need to run your application. So one consequence is: the lighter the faster.
Definition
The .slugignore file allows you to select files and directory which won’t be
include in the image after the build of your application. When you have heavy
build dependencies, you often don’t need them after the build, so why keeping
them? They would just slow down your application starting duration.
Usage
You just need to create a .slugignore file at the root of your project
containing on each line the name of a file or directory you want to exclude
from your image.
Example
Ruby application
Your ruby application doesn’t need to have all its test suites when running in production. You might want to exclude them during th deployment:
.slugignore content:
spec
Java application
Your application build process result in a .war
file and your don’t want to keep all the generated
classes and jar archives from the build:
.slugignore content:
.m2
Go application
You’re deploying a Go application but you don’t want to keep the Godeps directory containing all the third party dependency source code in the image:
.slugignore content:
Godeps