Add fmt check (with bad format to force a CI failure)

.gitlab-ci.yml

@@ -1,7 +1,16 @@
 ---
 stages:
+  - format
   - test
 
+check-fmt:
+  stage: format
+  image: nixos/nix:latest
+  script:
+    - nix-shell
+    - make fmt
+    - git diff --quiet
+
 run-tests:
   stage: test
   image: nixos/nix:latest

shell.nix

@@ -1,6 +1,7 @@
 with import <nixpkgs> {};
 mkShell {
   buildInputs = [
+    git
     go
     gotestsum
     gnumake