Extract before_script into a template that's not used for the docker build job

.gitlab-ci.yml

@@ -1,8 +1,8 @@
 variables:
   GIT_SUBMODULE_STRATEGY: recursive
 
-before_script:
-
+.before_script: &before_script
+  before_script:
     # Install dependencies
     # rsync is needed by opam to sync a package installed from a local directory with the copy in ~/.opam
     - apt-get update -qq
@@ -27,6 +27,7 @@ before_script:
     - opam switch
 
 local-dune-job:
+  <<: *before_script
   script:
     - vendors/ligo-opam-repository/rewrite-local-opam-repository.sh
     - opam repository add localrepo "file://$PWD/vendors/ligo-opam-repository-local/"
@@ -40,6 +41,7 @@ local-dune-job:
 #       - src/ligo/bin/cli.ml
 
 local-repo-job:
+  <<: *before_script
   script:
     - vendors/ligo-opam-repository/rewrite-local-opam-repository.sh
     - opam repository add localrepo "file://$PWD/vendors/ligo-opam-repository-local/"
@@ -47,6 +49,7 @@ local-repo-job:
     - opam install -y ligo
 
 remote-repo-job:
+  <<: *before_script
   script:
     # Add repository
     - opam repository add ligo-repository https://gitlab.com/gabriel.alfour/ligo-opam-repository.git