Well there could be an extension to the META format to specify an image name either in the support object or perhaps in a separate attribute, but I'd be worried that you'd spend all your time testing that rather than testing the module, also you'd end up re-inventing, say, "Github actions" as some of these things are going to be more than simply a docker container: a significant proportion of my modules, for example, install the external dependencies into the container as part of the test setup - otherwise you end up with a hulking great image. There are also those cases where the tests depend on an external service (say a DB server or something,) which may need configuring, starting etc. The complexity begins to stack up.

As a matter of fact, there's such a thing via the "from" field. Very few are using it, however. Also, there's no easy mapping from that to the actual package that can be installed. WRT to external services (as opposed to dependencies) I would strongly encourage people to mock them if they need to be installed... This is just ecosystem-inclusion testing, not production testing.