Add .gitlab-ci.yml

.gitlab-ci.yml

+image: fpco/stack-build:lts-14.6
+
+variables:
+  STACK_ROOT: "${CI_PROJECT_DIR}/.stack"
+
+cache:
+  paths:
+    - .stack-work
+    - .stack
+
+build:
+  stage: build
+  script:
+    - stack build
+    - cp $(stack path --dist-dir)/build/polygon/polygon .
+  artifacts:
+    paths:
+      - polygon