fix(ci): give the builder image what linuxdeploy needs for the AppImage

The v0.10.0 release build failed in Build Linux after 16 minutes:

  failed to bundle project: xdg-open binary not found
  /usr/bin/xdg-open: No such file or directory

linuxdeploy embeds xdg-open into the AppImage and aborts the whole bundle
when it is absent. deb and rpm had already bundled fine; only AppImage
was affected.

This is the one failure tonight that building locally could not have
caught, and the reason is worth writing down: a developer machine is a
desktop and always has xdg-utils, so the AppImage builds there and fails
on a minimal server image. The asymmetry is the bug. Every other release
defect this evening was found by building locally first; this one needed
the runner.

xdg-utils, desktop-file-utils and zsync are added together rather than
one at a time. Each round trip costs an image rebuild plus a failed
release build, and those three are what linuxdeploy commonly reaches for
(xdg-open, desktop-file-validate, and zsync for delta updates).

Workflows move to jellytau-builder:2026.08.1, built and pushed with all
three verified present inside it before this commit.

ci-operations.md gains two things learned here: that an apt addition
invalidates the layer above the cargo-install steps, so it is a ~20 minute
rebuild rather than the ~2 minutes the trailing layer normally gives; and
that Tauri's AppImage bundler downloads linuxdeploy, AppRun and two plugin
scripts from GitHub during the build, so an AppImage build depends on
GitHub being reachable from the runner.

.gitea/workflows/build-and-test.yml

@@ -28,7 +28,7 @@ jobs:
     if: "!startsWith(github.event.head_commit.message, 'chore(release)')"
     runs-on: linux/amd64
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
 
     steps:
       - name: Checkout repository
@@ -187,7 +187,7 @@ jobs:
     runs-on: linux/amd64
     needs: test
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
     env:
       ANDROID_HOME: /opt/android-sdk
       ANDROID_SDK_ROOT: /opt/android-sdk
@@ -256,7 +256,7 @@ jobs:
     name: Supply Chain
     runs-on: linux/amd64
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
 
     steps:
       - name: Checkout repository

.gitea/workflows/build-release.yml

@@ -21,7 +21,7 @@ jobs:
     name: Run Tests
     runs-on: linux/amd64
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
@@ -94,7 +94,7 @@ jobs:
     runs-on: linux/amd64
     needs: test
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
@@ -235,7 +235,7 @@ jobs:
     # baked into the builder image. No toolchain installs here — the image has
     # cargo-xwin, clang/clang-cl, lld, llvm, nsis and the msvc target.
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
@@ -308,7 +308,7 @@ jobs:
     runs-on: linux/amd64
     needs: test
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
     env:
       ANDROID_HOME: /opt/android-sdk
       ANDROID_SDK_ROOT: /opt/android-sdk
@@ -411,7 +411,7 @@ jobs:
     needs: [build-linux, build-windows, build-android]
     if: startsWith(github.ref, 'refs/tags/v')
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4

.gitea/workflows/publish-docs.yml

@@ -21,7 +21,7 @@ jobs:
     name: Build & publish docs to gitea-pages
     runs-on: linux/amd64
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
 
     steps:
       - name: Checkout code

.gitea/workflows/traceability-check.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: linux/amd64
     name: Check Requirement Traces
     container:
-      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08
+      image: gitea.tourolle.paris/dtourolle/jellytau-builder:2026.08.1
 
     steps:
       - name: Checkout repository

Dockerfile.builder

@@ -141,6 +141,17 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     lld \
     llvm \
     nsis \
+    # AppImage bundling. linuxdeploy embeds xdg-open into the AppImage and
+    # aborts the whole bundle if it is missing:
+    #   failed to bundle project: xdg-open binary not found
+    # It is present on most desktop distros, which is why the AppImage built on
+    # a developer machine and failed here. desktop-file-utils and zsync are the
+    # other two linuxdeploy commonly wants (desktop-file-validate, and zsync for
+    # delta updates), added together so a missing one does not cost another
+    # image rebuild and another failed release build.
+    xdg-utils \
+    desktop-file-utils \
+    zsync \
     && rm -rf /var/lib/apt/lists/* \
     # Ubuntu's clang package ships clang but NOT the clang-cl alias that cc-rs
     # invokes for MSVC targets. clang-cl is the same binary in MSVC-compat mode,

docs/build/ci-operations.md

@@ -61,6 +61,11 @@ filled. Keep a couple of dated tags live and prune the rest.
 
 The order matters — CI breaks if the workflow lands before the image exists.
 
+A caveat learned the hard way: the *trailing* layer is only fast for `cargo
+install` tools. Adding an **apt** package invalidates the packaging layer, which
+sits above the `cargo-xwin`/`cargo-deny` installs, so those recompile too — a
+~20 minute rebuild rather than ~2.
+
 ```bash
 # 1. Edit Dockerfile.builder. Put new tools in the TRAILING layer: it exists so
 #    a tool change is a ~2 min rebuild instead of ~15.
@@ -103,6 +108,25 @@ transitive upgrade (bumping `tauri-plugin-log` to 2.9.0 also moved `wry`,
 therefore video playback. That is a change to make deliberately, with a full
 build and a playback check — not one to slip into a release.
 
+## AppImage needs more than the Rust toolchain
+
+`linuxdeploy` (which Tauri downloads at build time to assemble the AppImage)
+shells out to distro tools that a minimal server image does not have. It aborts
+the whole bundle on the first one missing:
+
+```
+failed to bundle project: xdg-open binary not found
+```
+
+The image therefore carries `xdg-utils`, `desktop-file-utils` and `zsync`. This
+is a class of failure that **cannot be caught by building locally**: a developer
+machine is a desktop and has all three, so the AppImage builds there and fails in
+CI. It cost one release build to find.
+
+Tauri's AppImage bundler also downloads `linuxdeploy`, `AppRun` and two plugin
+scripts from GitHub during the build. That is Tauri's behaviour, not ours, but it
+means an AppImage build depends on GitHub being reachable from the runner.
+
 ## Secrets
 
 Managed with the `tea` CLI (`tea actions secrets list`) or the repo settings UI.