Squash of CI updates

.gitlab-ci.yml

 build-doc:
-    image: gitlab-research.centralesupelec.fr:4567/2017bentrioum/markovprocesses.jl/julia
-    stage: build
-    script:
-        - "julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd(), subdir=\"core/\")); Pkg.instantiate()'"
-        - "julia --project=docs/ docs/make.jl"
+  image: 2017bentrioum/markovprocesses.jl/julia:latest
+  stage: build
+  script:
+    - "docker image ls" 
+    - "julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd(), subdir=\"core/\")); Pkg.instantiate()'"
+    - "julia --project=docs/ docs/make.jl"
+