Add _IS_DEVCONTAINER environment variable

This will help my dotfiles identify if they're in a devcontainer.

Author: csutter

src/personal-setup/devcontainer-feature.json

@@ -1,6 +1,6 @@
 {
   "id": "personal-setup",
-  "version": "1.3.0",
+  "version": "1.4.0",
   "name": "Personal Setup",
   "description": "Sets up my desired software and configuration for any devcontainer environment.",
   "mounts": [
@@ -16,7 +16,8 @@
     }
   ],
   "containerEnv": {
-    "RCRC": "/mnt/dotfiles/tag-devcontainer/rcrc"
+    "RCRC": "/mnt/dotfiles/tag-devcontainer/rcrc",
+    "_IS_DEVCONTAINER": "true"
   },
   "postCreateCommand": "rcup -vf"
 }

test/personal-setup/test.sh

@@ -10,5 +10,6 @@ check "tree is available"       bash -c "which tree"
 check "ssh is available"        bash -c "which ssh"
 # Use a dotfile that should _always_ be present on any system
 check "dotfiles are installed"  bash -c "test -f $HOME/.config/fish/config.fish"
+check "_IS_DEVCONTAINER is set" bash -c "test \"\$_IS_DEVCONTAINER\" = \"true\""
 
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