Add DevContainer config (#17764)

* Add DevContainer config

* Add `--path vendor/bundle` to `postCreateCommand`

.devcontainer/devcontainer.json

@@ -0,0 +1,26 @@
+{
+  "name": "Mastodon",
+  "dockerComposeFile": "docker-compose.yml",
+  "service": "app",
+  "workspaceFolder": "/workspaces/mastodon",
+
+  // Set *default* container specific settings.json values on container create.
+	"settings": {},
+
+  // Add the IDs of extensions you want installed when the container is created.
+	"extensions": [
+    "EditorConfig.EditorConfig",
+    "dbaeumer.vscode-eslint",
+		"rebornix.Ruby"
+	],
+
+  // Use 'forwardPorts' to make a list of ports inside the container available locally.
+	// This can be used to network with other containers or the host.
+	"forwardPorts": [3000, 4000],
+
+	// Use 'postCreateCommand' to run commands after the container is created.
+	"postCreateCommand": "bundle install --path vendor/bundle && yarn install && ./bin/rails db:setup",
+
+	// Comment out to connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
+	"remoteUser": "vscode"
+}