Add gpg to Roundcube

The web UI was complaining that it couldn't find the GPG binary. Turns out it wasn't installed.

webmails/roundcube/Dockerfile

@@ -21,6 +21,7 @@ ENV ROUNDCUBE_URL https://github.com/roundcube/roundcubemail/releases/download/1
 RUN apt-get update && apt-get install -y \
       zlib1g-dev libzip4 libzip-dev \
       python3-jinja2 \
+      gpg \
  && docker-php-ext-install zip \
  && echo date.timezone=UTC > /usr/local/etc/php/conf.d/timezone.ini \
  && rm -rf /var/www/html/ \