Docker: add fpm to build container

docker/Dockerfile

@@ -450,6 +450,16 @@ RUN wget https://salsa.debian.org/klausenbusk-guest/debootstrap/commit/a9a603b17
     dpkg-buildpackage -us -uc && \
     sudo dpkg -i ../debootstrap*.deb
 
+#
+# fpm: a command-line program designed to help you build packages (e.g. deb)
+#
+RUN apt-get update && apt-get install -y \
+      ruby \
+      ruby-dev \
+      rubygems \
+      build-essential
+RUN gem install --no-document fpm
+
 # Allow password-less 'sudo' for all users in group 'sudo'
 RUN sed "s/^%sudo.*/%sudo\tALL=(ALL) NOPASSWD:ALL/g" -i /etc/sudoers && \
     chmod a+s /usr/sbin/useradd /usr/sbin/groupadd /usr/sbin/gosu /usr/sbin/usermod