Docker — Container Image Caching

A little container knowledge every day!

Docker caches existing images and reuses them when building new images. During the build process, if a certain image layer already exists, it will be used directly without recreating it.

Image Caching

Let’s take a look at the following example:

FROM ubuntu
RUN apt-get update && apt-get install -y vim
COPY testfile /

Build output:

$ docker build -t ububtu-with-vim .
Sending build context to Docker daemon  3.072kB
Step 1/3 : FROM ubuntu
 ---> 1f6ddc1b2547
Step 2/3 : RUN apt-get update && apt-get install -y vim
 ---> Using cache
 ---> ff719f2880f9
Step 3/3 : COPY testfile /
 ---> 0f3ffa9582cd
Successfully built 0f3ffa9582cd
Successfully tagged ububtu-with-vim:latest

Pay attention to this line:

Step 2/3 : RUN apt-get update && apt-get install -y vim
 ---> Using cache

Since the same RUN the command has been executed before, this time it is used directly. Then the process is to start the temporary container, copy the testfile, submit the new image layer, and delete the temporary container.

If you run the same docker build command with the same Dockerfile , the output will look like this: