CI: tag GWDG-related jobs with "schulz.com.de"

Unfortunately, GWDG machines are not accessible worldwide, so that runners.schulz.com.de is whitelisted^^

.gitlab-ci.yml

@@ -82,6 +82,9 @@ image: debian:bullseye-slim
 .configure_gwdg_template:
     extends: .run_template
     stage: configure
+    tags:
+        # run on runners.schulz.com.de because SSH is not open worldwide^^
+        - schulz.com.de
     variables:
         WORK_DIR: "/usr/local/snet-asclepios-deployment"
         COMMAND: |
@@ -158,6 +161,9 @@ image: debian:bullseye-slim
 .start-gwdg-template:
     extends: .run_template
     stage: start
+    tags:
+        # run on runners.schulz.com.de because SSH is not open worldwide^^
+        - schulz.com.de
     variables:
         WORK_DIR: "/usr/local/snet-asclepios-deployment"
         COMMAND: |
@@ -231,6 +237,9 @@ start-UoW:
 .test-gwdg-template:
     extends: .run_template
     stage: test
+    tags:
+        # run on runners.schulz.com.de because SSH is not open worldwide^^
+        - schulz.com.de
     variables:
         WORK_DIR: "/usr/local/snet-asclepios-deployment"
         COMMAND: |