Sven Luijten

Copying files from Docker Hub into your own image

Published on January 22nd, 2021 2 minutes to read

Did you know you can copy files from an image hosted on Docker Hub directly from your very own Dockerfile? In this post, I will use PHP's dependency manager Composer to show you how.

First, let me show you how you would do it without the method I am about to show you. Installing Composer programmatically can be quite arduous, and most people will likely do it this way in their Dockerfile:

FROM php:7

RUN apt-get update && apt-get -y install zip unzip

RUN php -r "copy('', 'composer-setup.php');"
RUN php -r "if (hash_file('sha384', 'composer-setup.php') === file_get_contents('')) { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
RUN php composer-setup.php
RUN php -r "unlink('composer-setup.php');"

RUN mv composer.phar /usr/local/bin/composer

# ...

Looks pretty decent, right? Well, what if I told you you could remove all those RUN statements, and replace them with a single COPY statement? It would end up looking like this:

FROM php:7

COPY --from=composer:2 /usr/bin/composer /usr/local/bin

# ...

Instead of installing the zip and unzip binaries, downloading the installer from, checking the file hash, running and removing the installer, and eventually moving the composer.phar to the /usr/local/bin directory, this single COPY statement copies the already built composer binary out of the official Composer Docker image, and moves it to /usr/local/bin in your container.

You can of course do this with any other Docker container available on Docker Hub. To do this, replace composer:2 from the snippet above with the name and tag of the image you want to copy out of.

The official Docker documentation does mention this feature, but it appears as though not a lot of people know about it (yet). I hope this post has helped you slim down your Dockerfile!