Add ruby-docker image for building docker images

.gitlab-ci.yml

@@ -40,6 +40,7 @@ xenial test: *test_build
 opensuse13.2 test: *test_build
 opensuse42.1 test: *test_build
 jessie_packer test: *test_build
+ruby_docker test: *test_build
 wheezy: *build_and_deploy
 centos7: *build_and_deploy
@@ -54,3 +55,4 @@ xenial: *build_and_deploy
 opensuse13.2: *build_and_deploy
 opensuse42.1: *build_and_deploy
 jessie_packer: *build_and_deploy
+ruby_docker: *build_and_deploy

docker/Dockerfile_ruby_docker

+FROM debian:jessie-slim
+
+# Install required packages
+RUN echo "deb http://ftp.debian.org/debian jessie-backports main" >> /etc/apt/sources.list \
+        && apt-get update -q \
+        && DEBIAN_FRONTEND=noninteractive apt-get install -yq --no-install-recommends \
+        ca-certificates \
+        git \
+        make \
+        gcc \
+        ruby \
+        ruby-dev \
+        libc6-dev \
+        docker.io \
+        && apt-get clean \
+        && rm -rf /var/lib/apt/lists/*
+
+RUN gem install bundler