add `.gitlab-ci.yml`

.gitlab-ci.yml

+stages:
+  - build
+
+build-frontend:
+  stage: build
+  image: node:latest
+  tags:
+    - docker
+  before_script:
+    - npm i -g elm sass
+  script:
+    - |
+      cd assets
+      elm make src/Main.elm --output=main.js
+      sass style.scss:style.css
+  after_script:
+    - |
+      mkdir dist/assets
+      cp main.js dist/assets
+      cp style.css dist/assets
+      cp index.html dist/assets
+  artifacts:
+    paths:
+      - dist/
+
+build-binaries:
+  stage: build
+  image: ocamlpro/ocaml:4.12
+  tags:
+    - docker
+  dependencies:
+    - build-frontend
+  before_script:
+    - sudo apk add bash openssl-libs-static
+  script:
+    - |
+      opam switch create . --deps ocaml-system
+      opam exec -- dune build --profile=static
+  after_script:
+    - |
+      mkdir dist
+      cp _build/default/bin/*.exe dist
+  artifacts:
+    paths:
+      - dist/
+