Thursday, January 25, 2018

Decrease Docker build time by using .dockerignore

Similar to the .tfignore file I blogged about yesterday, a .dockerignore file exists for Docker.

Why is this usefull?

To answer this question I have to explain a little bit how Docker works. When you are using Docker commands, there are in fact 2 components at work; a client and a daemon. The client only sends the commands to the daemon whereas the daemon does all the ‘real’ work.

One of the commands you probably used is docker build which allows you to build an image from a Dockerfile. When you execute this command the client will send all the files in the directory passed to the command to the daemon. For big projects, this can get very large resulting in slow builds as you have to wait until the client is done sending all files to the daemon.

Most of the time not all files are needed by docker build(e.g. previous bin and obj folders, your .git folder, …).  To avoid sending all these files you can create a .dockerignore file in your root directory. This works like a .gitignore file, listing the directories and files that Docker should ignore when creating the context, for example:

This can make a big difference even for small projects!

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