ostree-ext/cli: add --digestfile to ostree container image pull

This is something that I've wanted many times. There's no easy way from
e.g. bash to convert an image ref into the escaped OSTree ref and so
it's always awkward to be able to tell what ref/commit was pulled down.

Let's just match podman here and add a `--digestfile` switch for this.

Author: jlebon

ostree-ext/src/cli.rs

@@ -238,6 +238,10 @@ pub(crate) enum ContainerImageOpts {
         #[clap(value_parser = parse_imgref)]
         imgref: OstreeImageReference,
 
+        /// File to which to write the resulting OSTree commit digest
+        #[clap(long)]
+        ostree_digestfile: Option<Utf8PathBuf>,
+
         #[clap(flatten)]
         proxyopts: ContainerProxyOpts,
 
@@ -863,13 +867,15 @@ async fn container_info(imgref: &OstreeImageReference) -> Result<()> {
 async fn container_store(
     repo: &ostree::Repo,
     imgref: &OstreeImageReference,
+    ostree_digestfile: Option<Utf8PathBuf>,
     proxyopts: ContainerProxyOpts,
     quiet: bool,
     check: Option<Utf8PathBuf>,
 ) -> Result<()> {
     let mut imp = ImageImporter::new(repo, imgref, proxyopts.into()).await?;
     let prep = match imp.prepare().await? {
         PrepareResult::AlreadyPresent(c) => {
+            write_digest_file(ostree_digestfile, &c.merge_commit)?;
             println!("No changes in {} => {}", imgref, c.merge_commit);
             return Ok(());
         }
@@ -913,10 +919,19 @@ async fn container_store(
     if let Some(ref text) = import.verify_text {
         println!("{text}");
     }
+    write_digest_file(ostree_digestfile, &import.merge_commit)?;
     println!("Wrote: {} => {}", imgref, import.merge_commit);
     Ok(())
 }
 
+fn write_digest_file(digestfile: Option<Utf8PathBuf>, digest: &str) -> Result<()> {
+    if let Some(digestfile) = digestfile.as_deref() {
+        let rootfs = Dir::open_ambient_dir("/", cap_std::ambient_authority())?;
+        rootfs.write(digestfile.as_str().trim_start_matches('/'), digest)?;
+    }
+    Ok(())
+}
+
 /// Output the container image history
 async fn container_history(repo: &ostree::Repo, imgref: &ImageReference) -> Result<()> {
     let img = crate::container::store::query_image(repo, imgref)?
@@ -1095,12 +1110,14 @@ async fn run_from_opt(opt: Opt) -> Result<()> {
                 ContainerImageOpts::Pull {
                     repo,
                     imgref,
+                    ostree_digestfile,
                     proxyopts,
                     quiet,
                     check,
                 } => {
                     let repo = parse_repo(&repo)?;
-                    container_store(&repo, &imgref, proxyopts, quiet, check).await
+                    container_store(&repo, &imgref, ostree_digestfile, proxyopts, quiet, check)
+                        .await
                 }
                 ContainerImageOpts::Reexport {
                     repo,