Add ffmpeg buildpack for scalingo (#8500)

* Add ffmpeg buildpack for scalingo.

* Add environment variable to install ffprobe

Note: For existing applications, admins needs to add 'WITH_FFPROBE=true' to environment variable.

.buildpacks

 https://github.com/heroku/heroku-buildpack-apt
+https://github.com/Scalingo/ffmpeg-buildpack
 https://github.com/Scalingo/nodejs-buildpack
 https://github.com/Scalingo/ruby-buildpack

scalingo.json

@@ -91,6 +91,11 @@
       "description": "Internal scalingo configuration",
       "required": true,
       "value": "https://github.com/Scalingo/multi-buildpack.git"
+    },
+    "WITH_FFPROBE": {
+      "description": "Internal scalingo configuration to install ffprobe",
+      "required": true,
+      "value": "true"
     }
   },
   "scripts": {