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 environ

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.