Fix an issue where unmount would fail because of lingering processes

Update cleanup_image script to use fuser and umount --lazy --force in last
resort.

Fixes #105

.github/workflows/test-unmount-force.yml

@@ -0,0 +1,24 @@
+name: Test unmount succeeds even if processes are still running
+on:
+  push:
+    branches:
+      - 'main'
+      - 'releases/**'
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: ./ # pguyot/arm-runner-action@HEAD
+      with:
+        image_additional_mb: 1024
+        commands: |
+            sudo apt update -y
+            sudo apt install xvfb fluxbox -y
+            export DISPLAY=:0.0
+            Xvfb :0 -screen 0 1280x720x24 > /dev/null 2>&1 &
+            sleep 1
+            fluxbox > /dev/null 2>&1 &

cleanup_image.sh

@@ -26,12 +26,12 @@ if [[ -d "${mount}" ]]; then
         (cat /dev/zero >"${mount}/zero.fill" 2>/dev/null || true); sync; rm -f "${mount}/zero.fill"
     fi
 
-    umount "${mount}/dev/pts" || true
-    umount "${mount}/dev" || true
-    umount "${mount}/proc" || true
-    umount "${mount}/sys" || true
-    umount "${mount}/boot" || true
-    umount "${mount}" || true
+    umount "${mount}/dev/pts" || fuser -ckv "${mount}/dev/pts" || umount --force --lazy "${mount}/dev/pts" || true
+    umount "${mount}/dev" || fuser -ckv "${mount}/dev" || umount --force --lazy "${mount}/dev" || true
+    umount "${mount}/proc" || fuser -ckv "${mount}/proc" || umount --force --lazy "${mount}/proc" || true
+    umount "${mount}/sys" || fuser -ckv "${mount}/sys" || umount --force --lazy "${mount}/sys" || true
+    umount "${mount}/boot" || fuser -ckv "${mount}/boot" || umount --force --lazy "${mount}/boot" || true
+    umount "${mount}" || fuser -ckv "${mount}" || umount --force --lazy "${mount}" || true
 
     if [[ "${optimize}x" == "x" || "${optimize}x" == "yesx" ]]; then
         rootfs_partnum=${rootpartition}