Fixes #3629: Updating package-based installations can break the front end.

.pkgr.yml

@@ -116,4 +116,5 @@ env:
   - ZAMMAD_WEBSOCKET_PORT=6042
 services:
   - postgres
+before_install: contrib/packager.io/preinstall.sh
 after_install: contrib/packager.io/postinstall.sh

contrib/packager.io/preinstall.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+#
+# packager.io preinstall script
+#
+
+#
+# Make sure that after installation/update there can be only one sprockets manifest,
+#   the one coming from the package. The package manager will ignore any duplicate files
+#   which might come from a backup restore and/or a manual 'assets:precompile' command run.
+#   These duplicates can cause the application to fail, however.
+#
+rm -f /opt/zammad/public/assets/.sprockets-manifest-*.json || true