I have created a couple different directories on my host machine as I try to learn about Docker just to keep my dockerfiles organized. My Dockerfile I just ran looks like this:
FROM crystal/centos
MAINTAINER crystal
ADD ./rpms/test.rpm ./rpms/
RUN yum -y --nogpgcheck localinstall /rpms/test.rpm
My actual rpm is only 1 GB. But when I try to do sudo docker build -t="crystal/test" ., I get sending build context to Docker daemon 3.5 GB. Is there something else that I'm unaware of as you continue to build Docker images? Is my memory accumulating as I build more images in my other directories on my host machine?
12 Answers
The Docker client sends the entire "build context" to the Docker daemon. That build context (by default) is the entire directory the Dockerfile is in (so, the entire rpms tree).
You can setup a .dockerignore file to get Docker to ignore some files. You might want to experiment with it.
Alternatively, you can move your rpms folder one directory level above your Dockerfile, and only symlink test.rpm into the Dockerfile's directory.
You’ll often want to add the .git folder to the .dockerignore which was the cause of a 150MB -> 5GB difference for some users in the comments here.
Update 2019
Starting from Docker v18.06 there is an option to use a new image builder called Build Kit.
It's pre-bundled with the Docker, no need to install anything. It's backward compatible with the Dockerfile syntax, no need to change the Dockerfile.
Legacy Docker Build vs New Docker BuildKit
Here is an example of building an image with a huge unused file in the build directory:
Legacy Docker Build:
$ time docker image build --no-cache .
Sending build context to Docker daemon 4.315GB
[...]
Successfully built c9ec5d33e12e
real 0m51.035s
user 0m7.189s
sys 0m10.712s
New Docker BuildKit:
$ time DOCKER_BUILDKIT=1 docker image build --no-cache .
[+] Building 0.1s (5/5) FINISHED
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 37B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
[...]
=> => writing image sha256:ba5bca3a525ac97573b2e1d3cb936ad50cf8129eedfa9 0.0s
real 0m0.166s
user 0m0.034s
sys 0m0.026s
The only change is the DOCKER_BUILDKIT=1 environment variable, the difference in time is huge.
