Add test, bump version for extraNixConfig (#280)

src/nix/devcontainer-feature.json

 {
     "id": "nix",
-    "version": "1.0.0",
+    "version": "1.1.0",
     "name": "Nix Package Manager",
     "documentationURL": "https://github.com/devcontainers/features/tree/main/src/nix",
     "description": "Installs the Nix package manager and optionally a set of packages.",  

test/nix/extra-config.sh

+#!/bin/bash
+set -e
+
+# Optional: Import test library bundled with the devcontainer CLI
+source dev-container-features-test-lib
+
+uid="$(id -u)"
+echo "Current user UID is ${uid}."
+if [ "${uid}" != "1000" ]; then
+    echo "Current user UID was adjusted."
+fi
+set +e 
+vscode_uid="$(id -u vscode)"
+set -e
+if [ "${vscode_uid}" != "" ]; then
+    echo "User vscode UID is ${vscode_uid}."
+    if [ "${vscode_uid}" != "1000" ]; then
+        echo "User vscode UID was adjusted."
+    fi
+fi
+nix_uid="$(stat /nix -c "%u")"
+echo "/nix UID is ${nix_uid}."
+
+cat /etc/os-release
+cat /etc/nix/nix.conf 
+
+# Feature-specific tests
+# The 'check' command comes from the dev-container-features-test-lib.
+check "nix-env" type nix-env
+check "First added line" grep -E '^extra-substituters = https://foo\.cachix\.org$' /etc/nix/nix.conf 
+check "Second added line" grep -E '^extra-trusted-public-keys = foo\.cachix\.org-1:bar=$' /etc/nix/nix.conf 
+
+# Report result
+# If any of the checks above exited with a non-zero exit code, the test will fail.
+reportResults &2>1
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test/nix/scenarios.json

@@ -89,5 +89,14 @@
                 "flakeUri": "github:nixos/nixpkgs/nixpkgs-unstable#hello"
             }
         }
+    },
+    "extra-config": {
+        "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
+        "remoteUser": "vscode",
+        "features": {
+            "nix": {
+                "extraNixConfig": "extra-substituters = https://foo.cachix.org,extra-trusted-public-keys = foo.cachix.org-1:bar="
+            }
+        }
     }
 }
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