[GEB] geb-testcontainers integration update

Hi All,

The geb-testcontainers integration is in a working state.
* All tests pass in CI, locally, and with a build container.
* The module jars are included in the build.

Most important changes from my original Geb-Container fork of Grails-Geb:
* System properties and classes with Grails in the name have been renamed.
* WebDriverContainerHolder updated to work from the build container.
* Added a Dockerfile based on the Geb ci-docker-image (gebish/ci:v13) 
[2] and added a docker-entrypoint.sh
* Changed build-in-docker.sh to start the container interactive instead 
of exiting after the grade task.  Also changed it to optionally take a 
complete gradle command or just start at bash.

Some of this maybe due to my environment (macOS M2 and Podman) rather 
than Docker but hopefully works for others or we may need other changes.
The interactive container helped during development.

There is a README.md at [1].

I have a branch in my sample app repo [3] setup to use a maven local of 
geb-testcontainers 8.0.2-SNAPSHOT that you can build.
It has to be a local build and not using the container. Probably can't 
write outside the mounted directory. That maybe something we can fix 
with the container.

TODO's on my list:
Catch up changes from Grails-Geb that haven't been incorporated yet like 
picking up At-Check waiting from GebConfig instead of a system property.

Question:
Should we change the packages from grails.plugin.geb or leave them alone?

Next I'll check with the Grails project what might be needed for them to 
make use of the module if they want to go that direction.

Hopefully you can give it a try and we can discuss what we need before 
merging it into master.

Please let me know if you have any questions.

[1]https://github.com/apache/groovy-geb/tree/task/geb-testcontainers/integration/geb-testcontainers
[2]https://github.com/geb/ci-docker-image
[3]https://github.com/cbmarcum/geb-container-sample/tree/geb-testcontainers

Best regards,
Carl