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