Before Docker 17.05 dropped mid-2017, installing software inside a custom Docker image followed the same process as installing it on the host.

For example, you can install composer to /usr/local/bin on a desktop by running the following curl command as root.

curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer

To install it in a custom Docker image just prepend "RUN" and stick it into a Dockerfile:

# Dockerfile
FROM php:7.4.1-fpm
# Install Composer
RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer

Watch

Ta-da!

It works, but it relies on curl and an internet connection. Docker 17.05 brought a cleaner, less curl-y, internet-y way.

Multi-stage builds and COPY

Simply replace the RUN instruction with the COPY instruction seen below.

You can see that we are copying from the composer image from /usr/bin/composer to /usr/bin/composer in the new image.

Now let's see it in action, (as documented at https://hub.docker.com/_/composer):

# Dockerfile
FROM php:7.4.1-fpm
# Install Composer
COPY --from=composer /usr/bin/composer /usr/bin/composer

Watch

Ta-da*2!

I hope this helps someone.