Dont use reserved foldername (#20582)

Co-authored-by: Effy Elden <effy@effy.space>

.devcontainer/devcontainer.json

@@ -2,7 +2,7 @@
   "name": "Mastodon",
   "dockerComposeFile": "docker-compose.yml",
   "service": "app",
-  "workspaceFolder": "/workspaces/mastodon",
+  "workspaceFolder": "/mastodon",
 
   // Set *default* container specific settings.json values on container create.
   "settings": {},

.devcontainer/docker-compose.yml

@@ -13,7 +13,7 @@ services:
         # Optional Node.js version to install
         NODE_VERSION: '16'
     volumes:
-      - ..:/workspaces/mastodon:cached
+      - ..:/mastodon:cached
     environment:
       RAILS_ENV: development
       NODE_ENV: development