Pipeline stage to detect failures in the schema migrations

Between the build stages and the API test stage, this insinuates a new "alembic test" stage. This stage does part of the work of the API test but simply tries to run the schema container. The exit code of the container becomes the exit code of the script, so if it fails, we weren't able to run schema migrations. This will make it possible to disambiguate schema problems from other problems.

I've added an intentional problem to the migrations to see the pipeline fail. After I see that, I'll fix it to see if the pipeline passes.