Ship the customizations as a Docker image

The Docker image is used in the initialization process, to copy
everything in the custom folder to the Gitea data dir (mounted as a
persistent volume). It is built using Packer and is based on the busybox
image, so we can use its minimalist shell system to copy files and set
permissions

script/build_customizations_image

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# fail fast
+set -e
+
+cd packer/
+packer build custom.json
+cd -