Fix healthcheck port, correct base image for building locally

The application starts on port 8080 due to that being the configuration of the upstream container, however I mistakenly set the healthcheck to port 80.

Looking through other containers on ironbank, it seems that it is common to build using ironbank/opensource in the Dockerfile but opensource in the hardening manifest. This seems to allow the container to build locally for me and build on ironbank, win/win!