Update .gitlab-ci.yml file

.gitlab-ci.yml

+image: docker:latest
+services:
+    - docker:dind
+variables:
+    LATEST_VER: "registry.gitlab.com/synchrotron-soleil/control-system/containers/dev-el6:latest"
+stages:
+    - Build image and Push to Docker Hub
+docker build and push:
+    stage: Build image and Push to Docker Hub
+    only:
+        - master
+    before_script:
+        - echo "$REGISTRY_PASSWORD" | docker login -u "$REGISTRY_USER" --password-stdin
+    script:
+        - docker info
+        - docker build -t $LATEST_VER .
+        - docker push $LATEST_VER