feat: add GitLab CI pipeline for multi-service Docker build

Build all 5 iloom services (gateway, auth, purchaser, textile, washing)
as Docker images and push to Harbor registry.
- main branch: tagged as main-<sha>
- git tag v*: tagged with version number

Co-Authored-By: Claude <noreply@anthropic.com>

.gitlab-ci.yml

@@ -0,0 +1,57 @@
+stages:
+  - build
+
+variables:
+  HARBOR_REGISTRY: harbor-in-k3s.cheverjohn.me
+  HARBOR_PROJECT: iloom
+  DOCKER_TLS_CERTDIR: "/certs"
+
+.build-template: &build-template
+  stage: build
+  image: docker:27
+  services:
+    - docker:27-dind
+  before_script:
+    - echo "$HARBOR_PASSWORD" | docker login $HARBOR_REGISTRY -u "$HARBOR_USER" --password-stdin
+  script:
+    - |
+      if [ -n "$CI_COMMIT_TAG" ]; then
+        TAG="$CI_COMMIT_TAG"
+      else
+        TAG="main-${CI_COMMIT_SHORT_SHA}"
+      fi
+    - docker build -t $HARBOR_REGISTRY/$HARBOR_PROJECT/$IMAGE_NAME:$TAG -f $DOCKERFILE_PATH .
+    - docker push $HARBOR_REGISTRY/$HARBOR_PROJECT/$IMAGE_NAME:$TAG
+  rules:
+    - if: $CI_COMMIT_BRANCH == "main"
+    - if: $CI_COMMIT_TAG =~ /^v/
+
+build-gateway:
+  <<: *build-template
+  variables:
+    IMAGE_NAME: iloom-gateway
+    DOCKERFILE_PATH: gateway/Dockerfile
+
+build-auth-service:
+  <<: *build-template
+  variables:
+    IMAGE_NAME: iloom-auth-service
+    DOCKERFILE_PATH: auth-service/Dockerfile
+
+build-purchaser-service:
+  <<: *build-template
+  variables:
+    IMAGE_NAME: iloom-purchaser-service
+    DOCKERFILE_PATH: purchaser-service/Dockerfile
+
+build-textile-service:
+  <<: *build-template
+  variables:
+    IMAGE_NAME: iloom-textile-service
+    DOCKERFILE_PATH: textile-service/Dockerfile
+
+build-washing-service:
+  <<: *build-template
+  variables:
+    IMAGE_NAME: iloom-washing-service
+    DOCKERFILE_PATH: washing-service/Dockerfile