feat: docker image run side containers (#3222)

Adds Docker client to official renovate image so that it can run side containers.

Dockerfile

@@ -100,6 +100,18 @@ RUN groupadd --gid 1000 ubuntu \
 
 RUN chmod -R a+rw /usr
 
+# Docker client and group
+
+RUN groupadd -g 999 docker
+RUN usermod -aG docker ubuntu
+
+ENV DOCKER_VERSION=18.09.2
+
+RUN curl -fsSLO https://download.docker.com/linux/static/stable/x86_64/docker-${DOCKER_VERSION}.tgz \
+  && tar xzvf docker-${DOCKER_VERSION}.tgz --strip 1 \
+                 -C /usr/local/bin docker/docker \
+  && rm docker-${DOCKER_VERSION}.tgz
+
 USER ubuntu
 
 # Pipenv