No reason we couldn’t implement it, but we haven’t, is why you can’t use it today 
Our already-pretty flexible CI env of “do whatever you want in the unix shell” is already a pretty huge thing to provide tech support on (leads to around 2/5 of our Support contacts, almost all about things that the customer has misconfigured rather than bugs in our system), and allowing people to bring their own docker image complicates things further. I suppose we could implement a “0 tech support” policy on custom containers to ease the operational cost, but while we can say anything we want - people can and do set their own expectations and expect more than that, judging from a decade in the industry, and in the gap between expectations and reality lies much sadness.
Anyway, we do have an open feature request for that, and I’ve added this thread to it so we will follow up here to let you know if the situation changes.