Add gitlab pipeline for docker image weekly build

.gitlab-ci.yaml

+default:
+  image: docker:20.10.16
+  services:
+    - docker:20.10.16-dind
+  before_script:
+    - docker info
+
+variables:
+  # When you use the dind service, you must instruct Docker to talk with
+  # the daemon started inside of the service. The daemon is available
+  # with a network connection instead of the default
+  # /var/run/docker.sock socket. Docker 19.03 does this automatically
+  # by setting the DOCKER_HOST in
+  # https://github.com/docker-library/docker/blob/d45051476babc297257df490d22cbd806f1b11e4/19.03/docker-entrypoint.sh#L23-L29
+  #
+  # The 'docker' hostname is the alias of the service container as described at
+  # https://docs.gitlab.com/ee/ci/services/#accessing-the-services.
+  #
+  # Specify to Docker where to create the certificates. Docker
+  # creates them automatically on boot, and creates
+  # `/certs/client` to share between the service and job
+  # container, thanks to volume mount from config.toml
+  DOCKER_TLS_CERTDIR: "/certs"
+
+build:
+  stage: build
+  script:
+    - cd docker-pikaur/ && docker build -t recolic/pikaur .