Use multi-stage build

docker/app/install-composer.sh

@@ -1,21 +0,0 @@
-#!/bin/sh
-
-# Copyright (c) Nils Adermann, Jordi Boggiano
-# Origin: https://github.com/composer/composer/blob/master/doc/faqs/how-to-install-composer-programmatically.md
-# Licence: MIT
-
-EXPECTED_SIGNATURE=$(php -r "echo trim(file_get_contents('https://composer.github.io/installer.sig'));")
-php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
-ACTUAL_SIGNATURE=$(php -r "echo hash_file('SHA384', 'composer-setup.php');")
-
-if [ "$EXPECTED_SIGNATURE" != "$ACTUAL_SIGNATURE" ]
-then
-    >&2 echo 'ERROR: Invalid installer signature'
-    rm composer-setup.php
-    exit 1
-fi
-
-php composer-setup.php --quiet
-RESULT=$?
-rm composer-setup.php
-exit $RESULT