CI: add version for Docker image

.gitlab-ci.yml

 stages:
   - build
 
+variables:
+  VERSION: "latest"
+
 build:
   stage: build
   script:
-    - docker build -t dcr.mfb.io/devops/nexenta_exporter:latest .
+    - docker build -t dcr.mfb.io/devops/nexenta_exporter:${VERSION} .
+    - docker push dcr.mfb.io/devops/nexenta_exporter:${VERSION}
+    - docker tag dcr.mfb.io/devops/nexenta_exporter:${VERSION} dcr.mfb.io/devops/nexenta_exporter:latest
     - docker push dcr.mfb.io/devops/nexenta_exporter:latest
   tags:
     - autoscaling