Fix misuse of GitLab CI after_script section

I had been misusing the after_script section to run the docker build commands. As a result, failures of docker build were being mis-reported as successful CI jobs.

According to the GitLab CI help, the after_script section is run regardless of whether the script section succeeds or fails, and presumably if the after_script section fails, it has no impact on whether the job as a whole is reported as a failure.

The after_script section is apparently not supposed to be used for critical functionality of the job, but for non-critical cleanup code.