Attempt to prevent zombie processes when run under systemd

When systemd restarts the transmission-openvpn container, it seems to
leave zombie processes as below.

    $ ps uaxww | grep Z
    root      1872  0.0  0.0      0     0 ?        Z    08:58   0:00 [transmission-da] <defunct>

The zombies do appear to be cleaned up by restarting docker itself, or
by stopping the transmission-openvpn service.

This is an attempt to prevent zombies being spawned via Yelp's
dumb-init, which correctly handles the signals that PID1 gets.

See: https://github.com/Yelp/dumb-init

Dockerfile

@@ -16,6 +16,9 @@ RUN apt-get update \
     && apt-get update \
     && apt-get install -y transmission-cli transmission-common transmission-daemon \
     && apt-get install -y openvpn curl rar unrar zip unzip \
+    && curl -sLO https://github.com/Yelp/dumb-init/releases/download/v1.0.1/dumb-init_1.0.1_amd64.deb \
+    && dpkg -i dumb-init_*.deb \
+    && rm -rf dumb-init_*.deb \
     && apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* \
     && curl -L https://github.com/jwilder/dockerize/releases/download/v0.0.2/dockerize-linux-amd64-v0.0.2.tar.gz | tar -C /usr/local/bin -xzv
 
@@ -101,4 +104,4 @@ ENV OPENVPN_USERNAME=**None** \
 
 # Expose port and run
 EXPOSE 9091
-CMD ["/etc/openvpn/start.sh"]
+CMD ["dumb-init", "/etc/openvpn/start.sh"]