Add Ubuntu 20.04 build image

Dockerfile.omnibus-gitlab-focal

@@ -0,0 +1,24 @@
+FROM ubuntu:20.04
+
+RUN apt-get update -q \
+        && apt-get install -y g++ gcc git make curl zlib1g-dev libssl-dev locales \
+        && apt-get autoremove -yq \
+        && apt-get clean -yqq \
+        && 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 RUBY_VERSION 2.6.5
+RUN curl -fsSL "https://cache.ruby-lang.org/pub/ruby/2.6/ruby-${RUBY_VERSION}.tar.gz" \
+  | tar -xzC /tmp \
+  && cd "/tmp/ruby-${RUBY_VERSION}" \
+  && ./configure --disable-install-rdoc --disable-install-doc --disable-install-capi\
+  && make \
+  && make install \
+  && cd / \
+  && rm -rf "/tmp/ruby-${RUBY_VERSION}" \
+  && gem install bundler --no-document