How to install PHP composer inside a docker container

Question

I'm trying to figure out how to set up a development environment with Docker and Laravel.

The following is my 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
&& chmod -R o+rw laravel-master/bootstrap laravel-master/storage

When working with database migration, Laravel requires composer to perform composer dump-autoload. As a result, composer is required inside the Docker container.

I tried this but:

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

Later, when I call it

docker-compose up
docker-compose exec app composer dump-autoload

The following error is thrown:

rpc error: code = 13 desc = invalid header field value "oci runtime error: exec failed: container_linux.go:247: starting container process caused \"exec: \\\"composer\\\": executable file not found in $PATH\"\n"

I'd be grateful for any advise on how to add composer to the PATH in my dockerfile or what else I can do to get around this mistake.

Thank you for your help. Also, if you need to examine the docker-compose.yml file or anything else, go to the gitub repository(https://github.com/andrelandgraf/laravel-docker).