Fix wiping build root for --vm-type podman (#954)

* Refactor unmounting mounts under build root to unmount_build_root()

* Improve mount detection under the build root by adding a space prefix

Values in /proc/mounts are separated with a space.
Prepending the space to the searched pattern should reduce number of wrong matches

* podman: wipe remaining files from the build root

build

@@ -933,19 +933,22 @@ run_rsync() {
     fi
 }
 
-wipe_build_environment() {
-    if test -n "$VM_TYPE" ; then
-	vm_img_wipe
-    else
-	echo "Wiping build root: '$BUILD_ROOT'"
-
+unmount_build_root() {
 	# unmount all mounts still in the build root path
-	for m in $(cat /proc/mounts | grep "$BUILD_ROOT" | awk '{ print $2 }'); do
+	for m in $(cat /proc/mounts | grep " $BUILD_ROOT" | awk '{ print $2 }'); do
 	    if ! umount -n "$m" 2>/dev/null ; then
 		echo "Failed to umount "$m", cannot wipe buildroot"
 		exit 1
 	    fi
 	done
+}
+
+wipe_build_environment() {
+    if test -n "$VM_TYPE" ; then
+	vm_img_wipe
+    else
+	echo "Wiping build root: '$BUILD_ROOT'"
+	unmount_build_root
 	rm -rf "$BUILD_ROOT"
     fi  
 }

build-vm-podman

@@ -80,4 +80,11 @@ vm_sysrq_podman() {
 vm_wipe_podman() {
     local name="build_${RECIPEFILE}"
     podman rm "$name" >/dev/null 2>&1 || true
+
+    echo "Wiping build root: '$BUILD_ROOT'"
+    unmount_build_root
+    # calling 'podman unshare' is required because podman creates the files with SubUIDs/SubGIDs
+    # that differ from user's UID/GID and removing them would normally end up with
+    # a permission error for any user that is not root
+    podman unshare rm -rf "$BUILD_ROOT"
 }