fix(nix): remove the nix store cache causing rebuild inconsistencies

src/nix/NOTES.md

@@ -2,6 +2,27 @@
 
 This Feature should work on recent versions of Debian/Ubuntu, RedHat Enterprise Linux, Fedora, RockyLinux, and Alpine Linux.
 
+## Nix store persistence
+
+> [!WARNING]
+> On 1.3.x, Due to a nix cache `version`/`packages` changes are silently ignored on rebuild. `/nix` is mounted from a Docker volume that is seeded only on the first build. That mount can't be modified afterwards, so it keeps masking the newer image and no option change ever takes effect.
+
+**1.4.0+** removes the cache to avoid this; follow the workaround below to keep it.
+
+### Optional: cache `/nix` to speed up rebuilds
+
+Only if you use the default Feature (`"nix": {}`) and rebuild often: Add a local cache by pinning `/nix` to your own volume so runtime-installed packages survive recreation instead of being re-downloaded each time. Safe here because no options ever change.
+
+Add this as a top-level property in your `.devcontainer/devcontainer.json`:
+
+```json
+"mounts": [
+    { "source": "my-nix-store", "target": "/nix", "type": "volume" }
+]
+```
+
+Rebuild once to seed it. If you later add options or bump `version`, run `docker volume rm my-nix-store` first so the new image takes effect.
+
 ## Location of Flakes
 
 Currently `flakeUri` works best with a remote URI (e.g., `github:nixos/nixpkgs/nixpkgs-unstable#hello`) as local files need to be in the image.

src/nix/README.md

@@ -26,6 +26,27 @@ Installs the Nix package manager and optionally a set of packages.
 
 This Feature should work on recent versions of Debian/Ubuntu, RedHat Enterprise Linux, Fedora, RockyLinux, and Alpine Linux.
 
+## Nix store persistence
+
+> [!WARNING]
+> On 1.3.x, Due to a nix cache `version`/`packages` changes are silently ignored on rebuild. `/nix` is mounted from a Docker volume that is seeded only on the first build. That mount can't be modified afterwards, so it keeps masking the newer image and no option change ever takes effect.
+
+**1.4.0+** removes the cache to avoid this; follow the workaround below to keep it.
+
+### Optional: cache `/nix` to speed up rebuilds
+
+Only if you use the default Feature (`"nix": {}`) and rebuild often: Add a local cache by pinning `/nix` to your own volume so runtime-installed packages survive recreation instead of being re-downloaded each time. Safe here because no options ever change.
+
+Add this as a top-level property in your `.devcontainer/devcontainer.json`:
+
+```json
+"mounts": [
+    { "source": "my-nix-store", "target": "/nix", "type": "volume" }
+]
+```
+
+Rebuild once to seed it. If you later add options or bump `version`, run `docker volume rm my-nix-store` first so the new image takes effect.
+
 ## Location of Flakes
 
 Currently `flakeUri` works best with a remote URI (e.g., `github:nixos/nixpkgs/nixpkgs-unstable#hello`) as local files need to be in the image.

src/nix/devcontainer-feature.json

@@ -1,6 +1,6 @@
 {
     "id": "nix",
-    "version": "1.3.1",
+    "version": "1.4.0",
     "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.",
@@ -57,12 +57,5 @@
     "containerEnv": {
         "PATH": "/nix/var/nix/profiles/default/bin:/nix/var/nix/profiles/default/sbin:${PATH}"
     },
-    "mounts": [
-        {
-            "source": "nix-store-${devcontainerId}",
-            "target": "/nix",
-            "type": "volume"
-        }
-    ],
     "entrypoint": "/usr/local/share/nix-entrypoint.sh"
 }

test/nix/extra-config-packages.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+set -e
+
+source dev-container-features-test-lib
+
+check "Nix installation is not masked by a volume" bash -c "! mountpoint -q /nix"
+check "Experimental features config" grep -E '^experimental-features = nix-command flakes$' /etc/nix/nix.conf
+check "hello_installed" type hello
+check "hello_in_default_profile" bash -lc "nix-env -p /nix/var/nix/profiles/default -q | grep -q '^hello'"
+
+reportResults

test/nix/scenarios.json

@@ -108,5 +108,15 @@
                 "extraNixConfig": "extra-substituters = https://foo.cachix.org,extra-trusted-public-keys = foo.cachix.org-1:bar="
             }
         }
+    },
+    "extra-config-packages": {
+        "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
+        "remoteUser": "vscode",
+        "features": {
+            "nix": {
+                "extraNixConfig": "experimental-features = nix-command flakes",
+                "packages": "hello"
+            }
+        }
     }
 }