The Power of Docker’s COPY — from

I started working on a posting on multi-stage build after seeing a recent post about trying to reduce their container size with no mention of multi-stage builds. I was going to try and highlight the power of Docker’s COPY — from in that post but figured it would get lost.

In the Docker documents it says

Dockerfile reference

Optionally COPY accepts a flag — from=<name> that can be used to set the source location to a previous build stage (created with FROM .. AS <name>) that will be used instead of a build context sent by the user. In case a build stage with a specified name can’t be found an image with the same name is attempted to be used instead.

The final clause in that statement:

In case a build stage with a specified name can’t be found an image with the same name is attempted to be used instead.

is what I want to highlight. With that feature in the implementation of the COPY a docker file can pull in files from any accessible docker image.

To illustrate that feature I created a simple GitHub repo with a Dockerfile where it pulled in a file from an image that was not on my system (I did a docker system prune). Then during the run it reported resolving and retrieving the image from docker.io and then grabbing the file.

The examples can be seen at

GitHub - phutson/copyfromtest: An example of using Docker's copy from from an repo image

This provides another option for a data store that can be retrieved from during a build.

Important Points:

COPY — from

• Can copy from previous build stages
• Can copy from any retrievable docker image