Add CI to build docker image

.gitlab-ci.yml

+services:
+  - docker:dind
+
+before_script:
+  - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
+
+build:
+  only:
+    - schedules
+  stage: build
+  script:
+    - docker build -t $CI_REGISTRY_IMAGE .
+    - docker push $CI_REGISTRY_IMAGE

Dockerfile

+# Builds the decker executable
+FROM ubuntu:artful
+
+RUN apt-get update &&	apt-get install -y \
+  wget \
+  unzip \
+  zip
+
+RUN wget -qO- https://get.haskellstack.org/ | sh
+
+WORKDIR /decker
+COPY . /decker
+RUN make install
+
+# Image that will execute decker
+FROM ubuntu:artful
+
+RUN apt-get update && apt-get install -y \
+    graphviz \
+    gnuplot \
+    rsync \
+    sassc \
+    unzip \
+    zip
+
+ENV PATH="/root/.local/bin:${PATH}"
+COPY --from=0 /root/.local /root/.local
+COPY --from=0 /root/.stack /root/.stack
+
+#set the encoding on UTF-8, so the parser works correctly, german language is also added for umlaute
+#source of fix: https://blog.mkowalski.net/2016/05/16/solving-locale-issues-with-docker-containers/
+RUN apt-get install --reinstall -y locales && \
+	sed -i 's/# de_DE.UTF-8 UTF-8/de_DE.UTF-8 UTF-8/' /etc/locale.gen && \
+	locale-gen de_DE.UTF-8
+ENV LANG de_DE.UTF-8
+ENV LANGUAGE de_DE
+ENV LC_ALL de_DE.UTF-8
+RUN dpkg-reconfigure --frontend noninteractive locales
+
+WORKDIR /decker
+
+ENTRYPOINT ["decker"]