include build stage option

this MR adds the option to include a dedicated build stage. If one selects yes, then we will first create a -staging build, then a -test-build where the user can run tests using a postCommit hook, and then the real build

the -test-build is configured such that it is built with the contextDir docker/<name>/tests/ and uses the -staging-image as source (i.e. the -staging-image replaces the FROM part in the Dockerfile in docker/<name>/tests/)