Setting the `systemTest` Gradle task output as never up-to-date ensures
that all system tests are executed each time they are run in CI. The
`--rerun-tasks` Gradle option that was used previously had the same
effect but also disabled build caching.
Closes gh-29029
A new system test plugin is being made available for running test
suites that should be run less frequently than with every commit, such
as tests that verify Spring Boot compatibility with external
projects. CI pipeline configuration for running system tests is also
provided.
The first system tests verify the behavior of the Spring Boot image
building plugins when building images using Paketo buildpacks.
Closes gh-25824