Update .gitlab-ci.yml

.gitlab-ci.yml

 build-doc:
-  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"
-
+    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"