Support dumb-init on xenial

lib/worker_multibuild.py

@@ -75,9 +75,13 @@ dockerfile_extra_contents['xenial'] = """
 # Buildbot
 RUN apt-get update -y && apt-get install -yq --no-install-suggests --no-install-recommends \
     buildbot-slave git subversion python3-dev libffi-dev python3-setuptools \
-    python3-pip dumb-init curl openssh-client wget \
+    python3-pip curl openssh-client wget \
     python-setuptools python-psutil libssl-dev python-dev libgmp-dev python-virtualenv
 
+# dumb-init
+RUN wget https://github.com/Yelp/dumb-init/releases/download/v1.2.5/dumb-init_1.2.5_amd64.deb
+RUN dpkg -i dumb-init_*.deb && rm -f dumb-init_*.deb
+
 # buildbot entrypoint
 RUN wget -O /usr/local/bin/buildbot_entrypoint.sh https://git.bitmessage.org/Bitmessage/buildbot-scripts/raw/branch/master/docker/bionic/entrypoint.sh
 RUN chmod +x /usr/local/bin/buildbot_entrypoint.sh