add basic build pipeline

.gitlab-ci.yml

+variables:
+  DOCKER_HOST: "tcp://docker-dind.gitlab-suma:2375"
+  BUILD_DOCKER_IMAGE: docker:24.0.7
+  DEPLOY_KUBERNETES_IMAGE: alpine/k8s:1.22.6
+  KUBE_NAMESPACE: maps
+
+workflow:
+  rules:
+    - if: $CI_COMMIT_BRANCH == "master"
+      variables:
+        APP_URL: https://maps.metager.de
+        ENVIRONMENT: production
+        IMAGE_NAME: $CI_REGISTRY_IMAGE
+        IMAGE_TAG: master-$CI_COMMIT_SHA
+        HELM_RELEASE_NAME: $DOCKER_IMAGE_TAG_PREFIX
+
+stages:
+  - build
+
+build_images:
+  stage: build
+  before_script:
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+  script:
+    - docker compose build
+    - docker compose push
+  after_script:
+    - docker logout $$CI_REGISTRY
+