Add simple CI docker build using kaniko

.gitlab-ci.yml

+docker-build:
+  image:
+    name: gcr.io/kaniko-project/executor:v1.23.2-debug
+    entrypoint: [""]
+  stage: build
+  variables:
+    DOCKER_IMAGE_NAME: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG
+  script:
+    - /kaniko/executor
+      --context "${CI_PROJECT_DIR}"
+      --dockerfile "${CI_PROJECT_DIR}/Dockerfile"
+      --destination "${DOCKER_IMAGE_NAME}"
+  rules:
+    - if: $CI_COMMIT_BRANCH
+      exists:
+        - Dockerfile