Merge branch 'nrbffs/ci-cache' into 'master'

CI: add cache for builds

See merge request freifunk-region/stuttgart/site!4

.gitlab-ci.yml

@@ -24,6 +24,9 @@ stages:
     paths:
       - gluon/output/
     expire_in: 1 day
+  cache:
+    untracked: true
+    key: "$CI_COMMIT_TAG-$GLUON_TARGET"
  
 target:ar71xx-generic:
   extends: .target