Automate testing of the new image

Run automated tests when builidng to ensure all the features work as expected.

The tests do:

• Build a simple dockerfile
• Build a Dockerfile with files passed by context
• Use build args
• Override the FROM of the image

Some ideas to test in the future (this features have been manually tested):

• Pull and push a private image
• Multi stage builds (needs newer Docker than available in CC7)
• ARGs before FROM (needs newer Docker than available in CC7)

Also, this merge request assumes there is a runner tagged with docker-image-build-go pointing to runners with the new image.