Add a simple CI build test

.gitlab-ci.yml

+stages:
+  - test
+
+build-test:
+  stage: test
+  image: "registry.git.autistici.org/ai3/docker/test/golang:master"
+  script:
+    - apt-get -q update
+    - env DEBIAN_FRONTEND=noninteractive apt-get install -qy --no-install-recommends build-essential libfido2-dev
+    - go build -o webauthn-cred main.go