Limit memory available for a toolbox

I have a development container where I have some program that likes to eat up way too much memory. This has the tendency to bring the entire machine to a crawl till the offending program (or the entire container) is killed.

**Describe the solution you'd like**
I would like to set the maximum memory a toolbox can use.

**Describe alternatives you've considered**
What I do for a systemd-nspawn based container I use right now is to set `MemoryMax=16G` (or was it MaxMemory?) in the service file used to start the container. That works rather well.

No repository files or tests are named. Start by locating the toolbox container launch path and the relevant service or configuration entry point, using the reported systemd-nspawn MemoryMax=16G setting as context. Done means a toolbox can accept and enforce a configurable maximum memory limit.

Written by the indexing model from the issue text.