You can't run wifi-qcom-ac together with container on only 128mb ram. You will run into kernel panic reboots (or oom error)- guaranteed. But I just found what you mean, 7.21beta2 introduces this:
*) disk - allow to have type=file devices without rose-storage (needed for file based swap);
Hm, this could work. For some smaller containers and a rather small swap file.
But still the issue remains, unresolved: unknown disk consumption by ROS. There is already a topic: echo: system,error,critical could not save configuration changes, not enough storage space available.
16mb flash devices have a ram disk by default. ROS should not write any temporary files to flash. And if graphing persistence, lease persistence, etc. turned off - there should be nothing writing in an ideal world. ROS could utilize some part of the RAM for temporary files, even for nand devices this would be great.