add gitlab ci

Trying to add GitLab CI in order to release gcli binaries, but apparently the cache does not work.

.gitlab-ci.yml

+# Official language image. Look for the different tagged releases at:
+# https://hub.docker.com/r/library/rust/tags/
+image: "rust:latest"
+
+stages:
+  - build
+
+build:
+  stage: build
+  # image used for this build
+  image: rust
+  # build script
+  script:
+    - cargo build --release
+  # output file
+  artifacts:
+    paths:
+      - target/release/gcli
+  # use cache to avoid re-downloading and re-building all dependencies
+  cache:
+    - key:
+        files:
+          - Cargo.lock
+      paths:
+        - target/release
+  # only build gcli when adding a tag
+  only:
+    - tags
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