Switch to ai3/build-container rules for CI

.gitlab-ci.yml

-image: docker:latest
+include: "https://git.autistici.org/ai3/build-container/raw/master/common.yml"
 
-stages:
-  - build
-  - release
-
-services:
-  - docker:dind
-
-variables:
-  IMAGE_TAG: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME
-  RELEASE_TAG: $CI_REGISTRY_IMAGE:latest
-
-before_script:
-  - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN registry.git.autistici.org
-
-build:
-  stage: build
-  script:
-    - docker build --pull -t $IMAGE_TAG .
-    - docker push $IMAGE_TAG
-
-release:
-  stage: release
-  script:
-    - docker pull $IMAGE_TAG
-    - docker tag $IMAGE_TAG $RELEASE_TAG
-    - docker push $RELEASE_TAG
-  only:
-    - master