drop2beets - Docker mod for beets

This mod adds drop2beets to beets, to be installed/updated during container start.

In beets docker arguments, set an environment variable DOCKER_MODS=linuxserver/mods:beets-drop2beets

If adding multiple mods, enter them in an array separated by |, such as DOCKER_MODS=linuxserver/mods:beets-drop2beets|linuxserver/mods:beets-mod2