Merge branch 'ci-testing' into 'master'

CI testing

See merge request AirShowerPhysics/corsika!86

.gitlab-ci.yml

-image: ubuntu:bionic
+image: corsika/devel:u-18.04
 
 variables:
   GIT_SSL_NO_VERIFY: "1"
 
-before_script:
-  - apt-get update --yes
-  - apt-get install --yes cmake libboost-dev libeigen3-dev python3 gfortran
-  - apt-get install --yes doxygen graphviz
-
 build:
   stage: build
   tags:
@@ -16,7 +11,7 @@ build:
     - mkdir build
     - cd build
     - cmake ..
-    - cmake --build .
+    - cmake --build . -- -j 4 
     - ctest -j4 -V >& test.log
   after_script:
     - cd build