Dependabot won't update dependency due to closed PR, but PR can't be reopened

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Package ecosystem

Docker

### Package manager version

_No response_

### Language version

_No response_

### Manifest location and content before the Dependabot update

If possible I'd like to avoid posting internal code. However the relevant bit of `/Dockerfile` is

```dockerfile
FROM gcr.io/distroless/nodejs20-debian11:debug-nonroot@sha256:61ab5c263ca889340f2eca258f56b874ae0f82dec37222dda22e12f446441648
```

### dependabot.yml content

Sections for other package ecosystems have been omitted for brevity:

```yaml
version: 2
updates:
- package-ecosystem: "docker"
directory: "/"
schedule:
interval: "daily"
```

### Updated dependency

Original PR:
From: `gcr.io/distroless/nodejs20-debian11:debug-nonroot@sha256:1d4f76504fc495f074c86db1804c3a8325421d8dfa6e79680b2ad4a7ba8d646d`
To: `gcr.io/distroless/nodejs20-debian11:debug-nonroot@sha256:61ab5c263ca889340f2eca258f56b874ae0f82dec37222dda22e12f446441648`

Expected behavior now:
From: `gcr.io/distroless/nodejs20-debian11:debug-nonroot@sha256:61ab5c263ca889340f2eca258f56b874ae0f82dec37222dda22e12f446441648`
To: `gcr.io/distroless/nodejs20-debian11:debug-nonroot@sha256:10f36db40ca171cd929d90769531ccc39ad41116828f770b075aa67a1360d82f`

### What you expected to see, versus what you actually saw

Per the documentation and comments on #2024, if a Dependabot PR is closed, it will not open a new PR for that same version.

This causes issues with Docker images when the image digest changes but the tag version does not. Consider e.g. Google's distroless images, which always reuse tags such as `gcr.io/distroless/nodejs20-debian11:nonroot` -- updating by digest is the intended usage.

The normal workaround for this is to reopen the PR, at which point Dependabot will resume updating that dependency. However, the old PR may not necessarily be reopenable. Consider the following case:

1. Dependabot opens a PR to bump package xyz.
2. A developer realizes that additional changes need to be made in concert with the bump, and so creates another branch containing these changes.
1. Note that in this case, the exact commit from the Dependabot PR is present in the new branch.
3. The developer includes a comment in the manual PR "Closes #XYZ."
4. Manual PR merges, and the dependabot PR is marked as closed automatically by Github because of the comment.
5. Attempt to reopen the dependabot PR, but Github blocks this because "the commits are already merged."

I haven't been able to figure out a way around this, so for now we've just been doing updates manually. Any advice for a more convenient workaround would be welcome!

Also please let me know if this is something specific to the Github integrated version and we can instead raise the issue with support.

### Native package manager behavior

_No response_

### Images of the diff or a link to the PR, issue, or logs

_No response_

### Smallest manifest that reproduces the issue

_No response_

Start with the `/Dockerfile` digest example and the Docker section of `dependabot.yml`, then reproduce the workflow involving a closed Dependabot PR whose commit was merged through another PR. Done means establishing whether Dependabot can recognize the digest change and create a new update, or documenting a supported workaround if it cannot.

Written by the indexing model from the issue text.