Toolbx: Add unzip.

Makes the image useful for downloading and extracting files in a
multi-stage build.

toolbx/Dockerfile

@@ -20,6 +20,7 @@ RUN rm /etc/apt/apt.conf.d/docker-* && \
         locales \
         make \
         netbase \
+        unzip \
     && \
     export GNUPGHOME=/tmp/gnupghome && \
     mkdir --mode=0700 "$GNUPGHOME" && \