We recently allowed for customizable deployment runtimes for all the services. However, there is a big caveat in how we implemented this : the docker images associated with the default runtimes are built even if the user passes custom runtimes. In an of itself this is undesirable, but even more so because the way the CDK handles docker builds under the hood is kind of messy -- the less often it happens the better.
This ticket flags that we should make sure not to build our default images if custom runtimes are provided.