add default skin, use default skin as fallback

Dockerfile

@@ -88,10 +88,14 @@ RUN curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
 
 RUN mkdir -p /app/danser/custom-replays/
 
+RUN mkdir -p /app/danser/skins
+
 COPY src/maps/* /app/danser/songs/
 
 COPY src/replays/* /app/danser/custom-replays/
 
 COPY src/default.json /app/danser/settings/default.json
 
+COPY src/default-skin/ /app/danser/skins/
+
 WORKDIR /app/danser

src/default-skin/!LICENCE.txt

@@ -0,0 +1,7 @@
+All osu! graphic and other game assets are copyright ppy Pty Ltd.
+
+You are granted permission to use the elements contained within this archive as a template for creating your own skins. You can modify these in any way you see fit. Please do not include the resources if you haven't modified them; the defaults will automatically be used in this case.
+
+You are NOT permitted to use these graphics outside of skins and/or beatmaps. This includes using them on other websites, games, products etc.
+
+If you would like to use the resources outside of the scope provided above, please contact me at pe@ppy.sh

src/default-skin/!README.txt

@@ -0,0 +1,27 @@
+About this skin :
+=======================
+
+Template by Corne2Plum3 (https://osu.ppy.sh/users/15646039).
+
+This is the just the default skin from the stable version (no target practice sprites sorry...), but 2021 version (from the 20210821 version to be exact). I tried to make  the default skin but updated (the last I found was from 2014...), and complete (I added almost missing files) where skinners use it as a template to create their skin.
+
+Most of the sprites and gameplay sounds are from ppy/osu-ressources (https://github.com/ppy/osu-resources). Sometimes I used an AI to generates the @2x versions of some files. For the sounds, I tried to pick them directly from the game, so sorry if it's not accurate...
+
+Please don't kill me for making this, peppy!
+
+
+A few notes :
+=============
+
+BACK BUTTON:
+It's impossible to recreate the back button. In this skin, there is just 1 frame of the back button. Unlike the default, he's isn't animated, and doesn't change according to the game language.
+
+COMBO SPRITES:
+Same than score sprites.
+
+SLIDERBALL: 
+It's IMPOSSIBLE to recreate exactly the default sliderball with skinning, because sliderb files disable sliderb-spec (overlay) and sliderb-nd (background). So you will see only sliderb files in game with this skin.
+The only way to have the same sliderball than the default skin is deleting ALL slider ball files (sliderb, sliderb-spec and sliderb-nd).
+
+SPINNER APPROACH CIRCLE:
+With the default skin which uses new spinner, spinner-approachcircle is invisible. In the folder 'new spinner/visible approach circle' you have the default spinner-approachcircle if this file is needed.