Update all non-major dependencies Docker tags

Dockerfile

@@ -1,4 +1,4 @@
-FROM composer:1.7.2 AS composer
+FROM composer:1.9.1 AS composer
 
 FROM php:7-stretch
 

docker/docker-compose.7.yml

@@ -12,7 +12,7 @@ services:
       - phpserver
 
   phpserver:
-    image: php:7.0-fpm
+    image: php:7.4-fpm
     working_dir: /var/www/
     volumes:
       - ../:/var/www/