fix(scratch): add XDG_CACHE_HOME so peer cache is writable

Author: bethropolis

Dockerfile.scratch

@@ -21,7 +21,8 @@ EXPOSE 53317/udp
 # Point config and downloads to mounted volumes
 ENV LOCALSEND_DOWNLOAD_DIR="/app/downloads" \
     LOCALSEND_SECURITY_DIR="/app/config" \
-    LOCALSEND_AUTO_ACCEPT="true"
+    LOCALSEND_AUTO_ACCEPT="true" \
+    XDG_CACHE_HOME="/app/config/cache"
 
 # OCI labels
 LABEL org.opencontainers.image.title="LocalGo"