.gitlab-ci.yml: add ci jobs to build changed aports

Closes !74

.gitlab-ci.yml

 stages:
   - lint
+  - build
 
 # Make sure master points to the correct upstream commit
 before_script:
@@ -18,3 +19,42 @@ lint:
   tags:
     - docker-alpine
     - x86_64
+
+.build:
+  stage: build
+  image: alpinelinux/alpine-gitlab-ci:latest
+  script:
+    - build.sh
+  only:
+    - merge_requests
+
+build-x86_64:
+  extends: .build
+  tags:
+    - docker-alpine
+    - ci-build
+    - x86_64
+
+build-x86:
+  extends: .build
+  image:
+    name: alpinelinux/alpine-gitlab-ci:latest-x86
+    entrypoint: ["linux32", "sh", "-c"]
+  tags:
+    - docker-alpine
+    - ci-build
+    - x86
+
+build-s390x:
+  extends: .build
+  tags:
+    - docker-alpine
+    - ci-build
+    - s390x
+
+build-ppc64le:
+  extends: .build
+  tags:
+    - docker-alpine
+    - ci-build
+    - ppc64le