Add Gitlab CI/CD config

Add config file to set up Gitlab CI/CD for automatic container building

.gitlab-ci.yml

+docker-build:
+  # Use the official docker image.
+  image: docker:stable
+  stage: build
+  services:
+    - docker:26.0.1-dind
+  variables:
+    DOCKER_IMAGE_NAME: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG
+    DOCKER_TLS_CERTDIR: "/certs"
+  before_script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  cache:
+    key: ${CI_COMMIT_REF_SLUG}
+    paths:
+      - cache/
+  # All branches are tagged with $DOCKER_IMAGE_NAME (defaults to commit ref slug)
+  # Default branch is also tagged with `latest`
+  script:
+    - docker build --cache-from ${CI_REGISTRY_IMAGE}:${CI_COMMIT_REF_SLUG} --pull -t "$DOCKER_IMAGE_NAME" .
+    - docker push "$DOCKER_IMAGE_NAME"
+    - |
+      if [[ "$CI_COMMIT_BRANCH" == "$CI_DEFAULT_BRANCH" ]]; then
+        docker tag "$DOCKER_IMAGE_NAME" "$CI_REGISTRY_IMAGE:latest"
+        docker push "$CI_REGISTRY_IMAGE:latest"
+      fi
+  # Run this job in a branch where a Dockerfile exists
+  rules:
+    - if: $CI_COMMIT_BRANCH
+      exists:
+        - Dockerfile
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