fix(ci): pipeline only on merge request

Now runs the pipeline only once a MR is open.

Fixes #83

.gitlab-ci.yml

@@ -16,8 +16,13 @@ variables:
   DB_HOST: postgres
   DB_PORT: 5432
 
+.only-default: &only-default
+  only:
+    - master
+    - merge_requests
 
 check_back:
+  <<: *only-default
   stage: check
   image: registry.gitlab.utc.fr/rex-dri/rex-dri/backend
   services:
@@ -33,6 +38,7 @@ check_back:
     - docker
 
 check_front:
+  <<: *only-default
   stage: check
   image: node:10.15.1-alpine
   cache:
@@ -46,6 +52,7 @@ check_front:
     - docker
 
 test_back:
+  <<: *only-default
   stage: test
   image: registry.gitlab.utc.fr/rex-dri/rex-dri/backend
   services:
@@ -61,6 +68,7 @@ test_back:
     - docker
 
 test_frontend:
+  <<: *only-default
   stage: test
   image: node:10.15.1-alpine
   cache:
@@ -74,6 +82,7 @@ test_frontend:
     - docker
 
 flake8:
+  <<: *only-default
   stage: lint
   image: registry.gitlab.utc.fr/rex-dri/rex-dri/backend
   script:
@@ -82,6 +91,7 @@ flake8:
     - docker
 
 eslint:
+  <<: *only-default
   stage: lint
   image: node:10.15.1-alpine
   cache: