another approach

.gitlab-ci.yml

@@ -1,27 +1,18 @@
-before_script:
-  - docker info
-
-build_image:
-  script:
-    - docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN $CI_REGISTRY
-    - docker build -t $CI_REGISTRY_IMAGE:latest .
-    - docker push $CI_REGISTRY_IMAGE:latest
-  only:
-    - master
-  tags:
-    - shell
-    - x86
-
 image:
-  name: r.jalr.de/sbruder/packer-ansible
+  name: hashicorp/packer
+  entrypoint: [""]
+
+prepare:
+  script:
+    - apk add --no-cache ansible
 
 test:
   script:
-    - validate packer.json
+    - packer validate packer.json
 
 build:
   script:
-    - build packer.json
+    - packer build packer.json
   artifacts:
     paths:
     - rootfs.tar

Dockerfile

@@ -1,3 +0,0 @@
-FROM hashicorp/packer
-
-RUN apk add --no-cache ansible