[TASK] Prepare runTests.sh to flexible container bin

As a preparation towards podman as container
executor next to docker, runTests.sh is refactored
a bit:

* Image names now always have a host prefix like
  docker.io/ or ghcr.io/, except on CI, which will
  change later, too. podman needs those prefixes and
  does not fall back to docker.io if it's missing:
  "redis:latest" -> "docker.io/redis:latest"
* Image names are now put into variables.
* Rename "docker" to "container" in variables.
* Have a variable for the container binary, set to
  "docker" for now.
* Change the cleanUp() function slightly, it will
  later work with both docker and podman this way.

Resolves: #101470
Related: #97566
Releases: main, 12.4, 11.5
Change-Id: Ifab85468dff5c21564611a45568d11e8559e155c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80203
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>