ci: add .gitlab-ci.yml to automatically build the images

Signed-off-by: Vojtěch Mareš <vojtech@mares.cz>

.gitlab-ci.yml

@@ -0,0 +1,38 @@
+image:
+  name: hashicorp/packer:light
+  entrypoint: [""]
+
+variables:
+  PACKER_LOG: 1          # enable detailed logging
+  PACKER_VAR_FILE: "vars.pkrvars.hcl"
+
+stages:
+  - validate
+  - build
+
+.parallel:
+  parallel:
+    matrix:
+      - IMAGE: [ "flatcar" ]
+
+validate:
+  extends: .parallel
+  stage: validate
+  variables:
+    PACKER_WORKSPACE: images/$IMAGE
+  script:
+    - packer init $PACKER_WORKSPACE
+    - packer fmt -check $PACKER_WORKSPACE
+    - packer validate $PACKER_WORKSPACE
+
+build-image:
+  extends: .parallel
+  stage: build
+  variables:
+    PACKER_FILE: images/$IMAGE/image.hcl
+  script:
+    - |
+      packer build \
+        $PACKER_FILE
+  rules:
+    - if: $CI_COMMIT_BRANCH == "main"