How to install PHP composer inside a docker container

In Dockerfile :

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

I can install composer adding this line on my test dockerfile:

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

Here is the dockerfile:

FROM php:7.1.3-fpm

RUN apt-get update && apt-get install -y libmcrypt-dev \
    mysql-client libmagickwand-dev --no-install-recommends \
    && pecl install imagick \
    && docker-php-ext-enable imagick \
&& docker-php-ext-install mcrypt pdo_mysql

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

It works for me, to test if the composer are installed i access to my container bash and execute:

composer --version
Composer version 1.6.5 2018-05-04 11:44:59