Build context for a Dev Container Feature is invalidated on every build

# Current behavior

The build context for a Dev Container Feature is invalidated on every build, even when there are no changes to the `Containerfile`, `devcontainer.json`, or any files included in the build context.

As a result, the feature installation layer is rebuilt every time instead of reusing the existing cache.

# Expected behavior

The Dev Container Feature layer should reuse the build cache whenever possible and only be rebuilt when relevant inputs have changed.

# Environment

| Component | Version |
|-----------|---------|
| OS | Ubuntu 24.04 (Kernel 6.8.0-134-generic) |
| Arch | x86_64 |
| Container runtime | Podman 4.9.3 |
| VS Code | 1.112.0 |
| Dev Container CLI | 0.87.0 |

# Steps to reproduce

```shell
cd /tmp
git clone https://github.com/devcontainers/feature-starter.git
cd feature-starter
```

Remove the `docker-in-docker` feature and add the following feature to `devcontainer.json`:

```json
"ghcr.io/devcontainers/feature-starter/color:1": {
"favorite": "green"
}
```

Build the dev container:

```shell
devcontainer build --docker-path=/usr/bin/podman
```

Build it again without making any changes:

```shell
devcontainer build --docker-path=/usr/bin/podman
```

Notice that the layer responsible for installing the feature is rebuilt instead of being restored from cache:

```text
[2/2] STEP 5/9: RUN echo "_CONTAINER_USER_HOME=$( (command -v getent >/dev/null 2>&1 && getent passwd 'root' || grep -E '^root|^[^:]*:[^:]*:root:' /etc/passwd || true) | cut -d: -f6)" >> /tmp/dev-container-features/devcontainer-features.builtin.env && echo "_REMOTE_USER_HOME=$( (command -v getent >/dev/null 2>&1 && getent passwd 'node' || grep -E '^node|^[^:]*:[^:]*:node:' /etc/passwd || true) | cut -d: -f6)" >> /tmp/dev-container-features/devcontainer-features.builtin.env
--> Using cache c466b018334665827c5d83c2c00f910ce7f80e4bff6b0eb508ac254590faffba
--> c466b0183346

[2/2] STEP 6/9: RUN --mount=type=bind,from=dev_containers_feature_content_source,source=color_0,target=/tmp/build-features-src/color_0 \
cp -ar /tmp/build-features-src/color_0 /tmp/dev-container-features \
&& chmod -R 0755 /tmp/dev-container-features/color_0 \
&& cd /tmp/dev-container-features/color_0 \
&& chmod +x ./devcontainer-features-install.sh \
&& ./devcontainer-features-install.sh \
&& rm -rf /tmp/dev-container-features/color_0

===========================================================================
Feature : My Favorite Color
Description : A feature to remind you of your favorite color
Id : ghcr.io/devcontainers/feature-starter/color
Version : 1.0.3
Options :
FAVORITE="green"
===========================================================================

Activating feature 'color'
The provided favorite color is: green
...
...
```

# Additional observations

I am not sure exactly how feature tarballs are fetched and unpacked internally, but it appears that the feature tarball is downloaded on every invocation and extracted into a versioned directory under `/tmp` whose name changes each run.

For example:

```shell
$ ls -lh /tmp/devcontainercli-foo/container-features
total 52K
drwxrwxr-x 4 mv mv 4.0K Jul 6 19:15 0.87.0-1783358140200
drwxrwxr-x 4 mv mv 4.0K Jul 6 19:16 0.87.0-1783358186697
drwxrwxr-x 4 mv mv 4.0K Jul 6 19:17 0.87.0-1783358228464
```

This changing directory name is most likely to be contributing to cache invalidation.

Bắt đầu với bản tái hiện feature-starter, devcontainer.json của nó và entry point `devcontainer build --docker-path=/usr/bin/podman`. So sánh các build lặp lại và kiểm tra các thư mục tarball của feature dưới `/tmp/devcontainercli-foo/container-features`; hoàn thành khi một build không thay đổi sử dụng lại layer cài đặt feature, trong khi các thay đổi liên quan trong đầu vào vẫn làm layer đó mất hiệu lực.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.