Cache dither results

bin/server

@@ -3,9 +3,12 @@ set -euo pipefail
 
 repo_root=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
 dither_dir="${repo_root}/bin/dither"
+cache_dir="${DITHER_CACHE_DIR:-${repo_root}/.cache/dither}"
+
+mkdir -p "${cache_dir}"
 
 pushd "${dither_dir}" >/dev/null
-ROOT="${repo_root}/content" KEY="${repo_root}/secret.key" bundle exec ruby dither.rb &
+ROOT="${repo_root}/content" KEY="${repo_root}/secret.key" DITHER_CACHE_DIR="${cache_dir}" bundle exec ruby dither.rb &
 dither_pid=$!
 popd >/dev/null