chore: add devcontainer config (#228)

Author: johnstcn

.devcontainer/devcontainer.json

@@ -0,0 +1,9 @@
+{
+  "name": "terraform-provider-coder",
+  "image": "mcr.microsoft.com/devcontainers/go:1.20",
+  "features": {
+    "ghcr.io/devcontainers/features/terraform:1": {
+      "installTerraformDocs": true
+    }
+  }
+}