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('https://getcomposer.org/installer', 'composer-setup.php');"
RUN php -r "if (hash_file('sha384', 'composer-setup.php') === file_get_contents('https://composer.github.io/installer.sig')) { 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 getcomposer.org,
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!