devcontainer: drop redundant common-utils feature; ignore IDE build scratch

The MS Go devcontainer image (2.1-1.26) already ships vscode user,
sudo, git, curl, zsh, oh-my-zsh — the common-utils feature re-installs
all of them, stalling cold-start by ~2min behind deb.debian.org for no
behavioral change.

Also gitignore .devcontainer/.features.temp.dockerfile and friends —
JetBrains/VSCode's devcontainer builder writes those into the workspace
during image build and we don't want them in commits.

Author: css521

.devcontainer/devcontainer.json

@@ -16,14 +16,16 @@
     "--security-opt=apparmor=unconfined"
   ],
 
-  "features": {
-    "ghcr.io/devcontainers/features/common-utils:2": {}
-  },
-
+  // No `features` block: the MS Go devcontainer image already ships the
+  // vscode user, sudo, git, curl, zsh, oh-my-zsh, and everything else
+  // common-utils would have installed — adding the feature just re-runs
+  // a 2-minute apt-get for nothing (and stalls behind deb.debian.org on
+  // slow networks).
+  //
   // libbtrfs-dev lets you build with `-tags btrfs` (the optional fast-path
   // backend that needs cgo); the default static build needs nothing.
-  // sudo: devcontainer lifecycle commands run as remoteUser (vscode), not root.
-  // The common-utils feature gives vscode passwordless sudo.
+  // `sudo` because lifecycle commands run as remoteUser (vscode), and the
+  // base image's vscode user has passwordless sudo configured.
   "postCreateCommand": "sudo apt-get update && sudo apt-get install -y -qq libbtrfs-dev make",
   // Show the Makefile target list every time the container is attached, so a new
   // contributor immediately sees what they can do (build / test / verify / demo).

.gitignore

@@ -20,6 +20,12 @@ coverage.txt
 # Editor / OS
 .DS_Store
 .idea/
+# Devcontainer / JetBrains gateway scratch files generated during image build
+/.devcontainer/.features.temp.dockerfile
+/.devcontainer/.*.tmp
+# Devcontainer / JetBrains gateway scratch files generated during image build
+/.devcontainer/.features.temp.dockerfile
+/.devcontainer/.*.tmp
 # Allow committed shared settings, ignore per-user editor state.
 .vscode/*
 !.vscode/settings.json