aports: enable gitlab-ci linting

This will enable the CI linting job for merge requests done on gitlab.
The linting job will allow for failures, so linting failures will show
up as worning, but won't prevent the rest of the CI pipeline to
continue.

.gitlab-ci.yml

+stages:
+  - lint
+
+# Make sure master points to the correct upstream commit
+before_script:
+  - >
+    git fetch $CI_MERGE_REQUEST_PROJECT_URL
+    +refs/heads/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME:refs/heads/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME
+
+lint:
+  stage: lint
+  image: alpinelinux/apkbuild-lint-tools:latest
+  script:
+    - changed-aports $CI_MERGE_REQUEST_TARGET_BRANCH_NAME | lint
+  allow_failure: true
+  only:
+    - merge_requests
+  tags:
+    - docker-alpine
+    - x86_64