Add CI testing of as-built container

[iay]

I ran into an issue (now addressed) in which the version of logbook referenced in the POM was too old: it was explicit because the version in the Shibboleth parent was even older, and I needed to override that for Spring Boot 3.3.0 (I think). However, I hit a similar problem again with Spring Boot >= 3.3.3, which needed logbook 1.5.7.

This problem wasn't picked up by the unit tests for some reason, only by invoking the container. It seems, therefore, that we need to add testing of a built container to the CI in some way, on topic branches as well as on main. I'm not sure how to do that, however, as the container does not run to termination: I suppose we'd need to build a container within CI, fire it up in the background, send it a request and confirm that worked, then terminate the container.