On the other hand after asking the managers of an actively developed legacy system you might find bringing on new devs, or up new testing or integration environments is a major pain point.

That the skills to rebuild are in house - it's just that the process takes so long that bringing up a new environment stops other tasks. (The users here might say that bringing new features on line is taking too long)

In which dockerizing, at least for development, might be a very good thing.

But starting with the pain and talking to the managers and users is absolutely the right first step.