Build docker image only when necessary

.gitlab-ci.yml

@@ -6,7 +6,7 @@ variables:
   PACKER_VERSION: 1.4.3
   ANNOUNCE: http://labsync.lab.fablab-nea.de:6969/announce
   WEBSEED: http://labsync.lab.fablab-nea.de/labsync/$CI_COMMIT_REF_NAME/$CI_PIPELINE_ID/images
-  DOCKER_IMAGE_BUILDER: ${CI_REGISTRY_IMAGE}/labsync-builder:$CI_COMMIT_REF_SLUG
+  DOCKER_IMAGE_BUILDER: ${CI_REGISTRY_IMAGE}/labsync-builder:main
   DOCKER_IMAGE_SECURITY_SCANNER: ${CI_REGISTRY_IMAGE}/security-scanner:$CI_COMMIT_REF_SLUG
   DOCKER_TLS_CERTDIR: ""
 
@@ -21,14 +21,26 @@ dockerimage_builder:
     - apk add --no-cache make
   services:
    - docker:dind
+  variables:
+    DOCKER_IMAGE_BUILDER: ${CI_REGISTRY_IMAGE}/labsync-builder:$CI_COMMIT_REF_SLUG
   script:
     - docker pull $DOCKER_IMAGE_BUILDER || true
     - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
     - make builderimg
     - docker push $DOCKER_IMAGE_BUILDER
-  except:
-    refs:
-      - schedules
+    - echo "DOCKER_IMAGE_BUILDER=$DOCKER_IMAGE_BUILDER" >> build.env
+  artifacts:
+    reports:
+      dotenv: build.env
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
+      when: on_success
+    - if:
+      changes:
+        paths:
+          - builder
+        compare_to: main
+      when: on_success
 
 dockerimage_security_scanner:
   stage: prepare