Add composer

[matt-in-a-hat]

This image doesn't have composer installed right? Are you able to add it e.g. as done here

# Install composer for PHP dependencies
RUN cd /tmp && curl -sS https://getcomposer.org/installer | php && mv composer.phar /usr/local/bin/composer

[matt-in-a-hat]

The reasoning for requiring this is that composer checks the system for dependencies (php version, mcrypt, etc), which means that it should be run from within the container else will fail if the host system doesn't meet requirements (or worse - succeed on compose install but fail at runtime due to missing requirements within the container).