.gitlab-ci.yml: add "duneci" tag for shared runner [ci skip]

.gitlab-ci.yml

@@ -5,23 +5,29 @@ before_script:
 debian:10  gcc:c++17:
   image: duneci/base:10
   script: duneci-standard-test --opts=/duneci/opts.gcc.c++17
+  tags: [duneci]
 
 debian:9--gcc:
   image: duneci/base:9
   script: duneci-standard-test
+  tags: [duneci]
 
 debian:9--clang:
   image: duneci/base:9
   script: duneci-standard-test --opts=/duneci/opts.clang
+  tags: [duneci]
 
 debian:8--gcc:
   image: duneci/base:8
   script: duneci-standard-test
+  tags: [duneci]
 
 ubuntu:16.04--gcc:
   image: duneci/base:16.04
   script: duneci-standard-test
+  tags: [duneci]
 
 ubuntu:16.04--clang:
   image: duneci/base:16.04
   script: duneci-standard-test --opts=/duneci/opts.clang
+  tags: [duneci]