Use composer installer instead of docker image copy

Author: mRoca

docker/php-flex/Dockerfile

@@ -54,9 +54,9 @@ RUN set -eux; \
   \
   apk del .build-deps
 
-COPY --from=composer:1.10 /usr/bin/composer /usr/bin/composer
-
-RUN mkdir /.composer \
+RUN curl https://getcomposer.org/composer-stable.phar -o /usr/bin/composer \
+    && chmod +x /usr/bin/composer \
+    && mkdir /.composer \
     && chown -R www-data:www-data /.composer \
     && su-exec www-data composer global require "hirak/prestissimo" "jderusse/composer-warmup" --prefer-dist --no-progress --no-suggest --classmap-authoritative \
     && su-exec www-data composer clear-cache -n