add Dockerfile and GitLab CI configuration in order to build DPsim in a Docker container on the CI

.gitlab-ci.yml

+stages:
+  - prepare
+  - build
+
+docker:
+  stage: prepare
+  script:
+    - docker build -t dpsim-dev .
+  tags:
+    - shell
+
+build:
+  stage: build
+  script:
+    - make -C Source
+  image: dpsim-dev
+  tags:
+    - docker
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Dockerfile

+FROM fedora:latest
+MAINTAINER Steffen Vogel <stvogel@eonerc.rwth-aachen.de>
+
+RUN dnf -y install \
+	gcc-c++ \
+	make \
+	eigen3-devel \
+	doxygen
+	
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