Support ubuntu 18.04

Dockerfile.omnibus-gitlab-bionic

@@ -0,0 +1,23 @@
+FROM ubuntu:18.04
+
+RUN apt-get update -q \
+        && apt-get install -y g++-6 gcc-6 git make curl zlib1g-dev libssl1.0-dev locales \
+        && rm -rf /var/lib/apt/lists/*
+
+RUN echo "en_US.UTF-8 UTF-8" > /etc/locale.gen
+RUN locale-gen en_US.UTF-8
+ENV LANG en_US.UTF-8
+ENV LANGUAGE en_US:en
+ENV LC_ALL en_US.UTF-8
+ENV CC /usr/bin/gcc-6
+ENV CXX /usr/bin/g++-6
+
+ENV RUBY_VERSION 2.3.3
+RUN curl -fsSL "https://cache.ruby-lang.org/pub/ruby/2.3/ruby-${RUBY_VERSION}.tar.gz" \
+  | tar -xzC /tmp \
+  && cd /tmp/ruby-2.3.3 \
+  && ./configure --disable-install-rdoc --disable-install-doc --disable-install-capi\
+  && make \
+  && make install
+
+RUN gem install bundler