ci: Add GitLab CI/CD pipeline for container builds

Builds and pushes container images to GitLab Container Registry
when version tags (v*.*.*) are pushed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitlab-ci.yml

@@ -0,0 +1,40 @@
+# GitLab CI/CD Pipeline for StarPunk
+# Builds and pushes container images on version tags
+
+stages:
+  - build
+
+variables:
+  # Required for Docker-in-Docker
+  DOCKER_TLS_CERTDIR: "/certs"
+  # Use overlay2 driver for better performance
+  DOCKER_DRIVER: overlay2
+
+# Build and push container image
+build-container:
+  stage: build
+  image: docker:24
+  services:
+    - docker:24-dind
+  # Only trigger on version tags (e.g., v0.9.5, v1.0.0)
+  rules:
+    - if: $CI_COMMIT_TAG =~ /^v[0-9]+\.[0-9]+\.[0-9]+$/
+  before_script:
+    # Login to GitLab Container Registry
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+  script:
+    # Build the container image with both version and latest tags
+    - |
+      docker build \
+        --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG \
+        --tag $CI_REGISTRY_IMAGE:latest \
+        --file Containerfile \
+        .
+    # Push both tags to the registry
+    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
+    - docker push $CI_REGISTRY_IMAGE:latest
+  # Cache Docker layers between builds
+  cache:
+    key: docker-$CI_COMMIT_REF_SLUG
+    paths:
+      - .docker-cache