Ensure that package managers clean up

Every package manager should clean up after themselves in order to keep
docker layers neat and tiny:

apt (Debian/Ubuntu package manager):

 - unneeded dependencies are cleared (autoremove)
 - caches are cleaned (clean)
 - package lists are deleted

yum (CentOS package manager),
zypper (OpenSuse package manager)

 - should clear caches after installing dependencies

pip (Python package manager),
apk (Alpine package manager)

 - should use no cache for installing dependencies

Dockerfile.danger

@@ -6,6 +6,8 @@ ADD /scripts/ /scripts/
 RUN apt-get update \
     && apt-get install -y git \
     && /scripts/install-node 8.x 1.2.1-1 \
+    && apt-get autoremove -yq \
+    && apt-get clean -yqq \
     && rm -rf /var/lib/apt/lists/* \
     && gem install danger-gitlab --no-document \
     && git version \