ci: add docker build to gitlab ci/cd (#3)

Update the Gitlab CI/CD pipeline to include a step to build and push the
project docker image. The CI/CD step uses existing Makefile commands to
build the docker image.

.gitlab-ci.yml

@@ -35,3 +35,17 @@ build:
       - dist/*.tar.gz
       - dist/checksums.txt
     expire_in: 1 day
+
+docker-gitlab:
+  stage: build
+  image: docker:stable
+  services:
+    - docker:dind
+  before_script:
+    - apk add git
+    - apk add make
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+  script:
+    - make docker/build-latest
+    - make docker/build-tag
+    - docker push registry.gitlab.com/hectorjsmith/fail2ban-prometheus-exporter