The latest roundcube version was removed from sourceforge

Link to Github instead.

roundcube/Dockerfile

@@ -7,7 +7,7 @@ RUN apt-get update && apt-get install -y \
       libpng12-dev \
  && docker-php-ext-install pdo_mysql mcrypt
 
-ENV ROUNDCUBE_URL https://downloads.sourceforge.net/project/roundcubemail/roundcubemail/1.1.5/roundcubemail-1.1.5-complete.tar.gz
+ENV ROUNDCUBE_URL https://github.com/roundcube/roundcubemail/releases/download/1.1.5/roundcubemail-1.1.5-complete.tar.gz
 
 RUN echo date.timezone=UTC > /usr/local/etc/php/conf.d/timezone.ini