Add gitlab workflow to build docker image

Automatically run only on pushed tags, but can also be run manually at any time.

Change-Id: Iff54b5be7c24f3dcdc3912462d472abd523f4ac2
diff --git a/.gitignore b/.gitignore
index de38250..d0db035 100644
--- a/.gitignore
+++ b/.gitignore
@@ -42,6 +42,7 @@
 /t/kalamar_user_client.t
 !.gitignore
 !.github
+!.gitlab-ci.yml
 !.dockerignore
 /npm-debug.log
 /package-lock.json