fix(docker): always build on tag

!68 fixes #37

docker/build.gitlab-ci.yaml

@@ -107,6 +107,13 @@ variables:
         - '{dockerfile,dockerfile.j2}'
       when: always
 
+    - if: # condition_git_tag
+        $CI_COMMIT_TAG != null &&
+        $CI_COMMIT_BRANCH == null
+      exists:
+        - '{dockerfile,dockerfile.j2}'
+      when: on_success
+
     - if: # condition_dev_branch_push
         $CI_COMMIT_BRANCH == "development" && 
         $CI_PIPELINE_SOURCE == "push"