create helper tests for in-progress image PRs

[tobybellwood]

we've got in-progress PRs for mysql (#308) and dotnet (#550)

Neither of these images has a set of tests in the helper docker-compose stack. We should update each by adding it's images to the docker-compose, and then creating the additional tests - for MySQL, they can be similar to mariadb (although that may require some work with uselagoon/internal-services-test to support MySQL), and for dotnet, maybe there's a one-liner we can use to spin up a webserver (similar to python, node and ruby)

Note that these images are still a WIP, so can be changed if needed to support this.

(note, to test locally, replace the image: uselagoon/dotnet-6.0:latest with lagoon/dotnet-6.0:latest in the docker-compose)