Upgrade the Docker setup and dependencies

composer.json

@@ -3,17 +3,13 @@
     "type": "project",
     "license": "proprietary",
     "description": "Project description",
-    "minimum-stability": "dev",
     "require": {
-        "php": "^7.1",
-        "symfony/flex": "1.0.x-dev",
-        "symfony/framework-bundle": "3.3.x-dev",
-        "symfony/yaml": "3.3.x-dev"
+        "php": "^7.1.3",
+        "symfony/flex": "^1.0",
+        "symfony/framework-bundle": "^3.3",
+        "symfony/yaml": "^3.3"
     },
     "config": {
-        "platform": {
-            "php": "7.1"
-        },
         "preferred-install": {
             "*": "dist"
         },
@@ -39,11 +35,8 @@
             "@auto-scripts"
         ]
     },
-    "repositories": [
-        { "type": "vcs", "url": "https://github.com/fabpot/flex" }
-    ],
     "conflict": {
-        "symfony/symfony": "<3.3",
+        "symfony/symfony": "*",
         "symfony/twig-bundle": "<3.3",
         "symfony/debug": "<3.3"
     },