Build Labgrid container images (derived from config in yocto-ktn/ci-tests)

Adding the docker files and CI config to build the Labgrid docker image. This is derived from the config in yocto-ktn with some improvements for the pipeline. The goal is to remove the Labgrid images from yocto-ktn, as they don't really belong there.

.gitlab-ci.yml

@@ -38,6 +38,7 @@ generate-client-keys:
   artifacts:
     paths:
       - key/
+    expire_in: 1 day
 
 build:labgrid:client:
   extends: .build-docker