Update deploy image to use Dart 3.12.1

cloud_build/firebase-ghcli/Dockerfile

@@ -1,4 +1,4 @@
-FROM dart:3.12.0@sha256:3d1b19886b288807b42020ede8f1f9ea7753c91c17c27479ee9e7bf584289a34
+FROM dart:3.12.1@sha256:6440c7d5fd8713b0706d0b6190eb2be7ad896101e225fc9d7657034b23ab0592
 
 RUN apt-get update && apt-get install -y --no-install-recommends curl gpg ca-certificates \
     # Install the GitHub CLI. \

cloud_build/firebase-ghcli/README.md

@@ -18,3 +18,64 @@ When the `Dockerfile` file or `cloudbuild.yaml` template in this directory
 are changed in a PR, the cloud build template is triggered.
 Then a new version of the image is deployed as the latest version in
 Container Registry under the `flutter-dev-230821` project on Google Cloud.
+
+## Updating the Dart SDK version
+
+The base Dart image in `Dockerfile` is pinned by both tag and digest:
+
+```dockerfile
+FROM dart:<version>@<digest>
+```
+
+When updating the Dart SDK version, update both the tag and the digest.
+Use the multi-platform image index digest for the tag.
+Don't use a platform-specific digest, such as
+the digest for only `linux/amd64` or only `linux/arm64/v8`.
+
+### Using `crane`
+
+If you have the `crane` CLI from [`google/go-containerregistry`][], run:
+
+```bash
+crane digest docker.io/library/dart:<STABLE_DART_SDK_VERSION>
+```
+
+For example, the following command retrieves the digest for
+the `3.12.1` stable release of the SDK:
+
+```bash
+crane digest docker.io/library/dart:3.12.1
+```
+
+Copy the returned `sha256:...` digest.
+Then update the `FROM` line in the `Dockerfile` with
+the new tag (version) and copied index digest.
+
+```dockerfile
+FROM dart:3.12.1@<digest from crane>
+```
+
+Don't pass `--platform` when getting the digest for the `Dockerfile`.
+Passing a platform returns the digest for that platform's child manifest
+instead of the architecture-agnostic index digest.
+
+[`google/go-containerregistry`]: https://github.com/google/go-containerregistry
+
+### Using Docker Hub
+
+If you don't have `crane`, use the
+[Dart image tags page on Docker Hub](https://hub.docker.com/_/dart/tags):
+
+1.  Search for the new Dart SDK version tag, such as `3.12.1`.
+1.  Open the tag detail page by selecting the tag name or digest.
+1.  Ensure the opened tag page has a `MULTI-PLATFORM` badge.
+1.  Copy the specified `INDEX DIGEST` value.
+    It should start with `sha256:`.
+1.  Update the `FROM` line in the `Dockerfile` with
+    the new tag (SDK version) and index digest.
+
+<img
+  width="688"
+  height="248"
+  alt="Screenshot of the index digest on Docker Hub, outlined with a red box."
+  src="https://github.com/user-attachments/assets/909bd713-580f-41f5-882b-9afb3b9bbfaf" />