Add a base docker image for testing

This way we have a central place for the CI image config and also testing
is faster because not everythign has to be installed on each run.

packages/common/.gitlab-ci/Dockerfile

+FROM ubuntu:18.04
+
+ENV LANG C.UTF-8
+ENV DEBIAN_FRONTEND noninteractive
+
+RUN apt-get update && apt-get install -y \
+    git \
+    curl
+
+RUN curl -sL https://deb.nodesource.com/setup_12.x | bash -
+
+RUN apt-get update && apt-get install -y \
+    nodejs \
+    chromium-browser \
+    firefox \
+    composer \
+    rsync
\ No newline at end of file

packages/common/.gitlab-ci/README.md

+* In case of an incompatible change increase the tag version number in build.sh
+* ./build.sh
+* (first time) sudo docker login registry.gitlab.tugraz.at
+* sudo docker push registry.gitlab.tugraz.at/vpu/webcomponents/common/main:v1
\ No newline at end of file

packages/common/.gitlab-ci/build.sh

+#!/bin/bash
+
+set -e
+TAG="registry.gitlab.tugraz.at/vpu/webcomponents/common/main:v1"
+sudo docker build --tag "${TAG}" --file "Dockerfile" .
+echo "Now run: sudo docker push '$TAG'"