Overlapping mounts don't work correctly

I wanted to keep `~` readonly, but make another directory below it (say, `~/tmp`) writable. I get one of 2 results:

1. `~/tmp` is still readonly, all of `~` is visible, or
2. `~/tmp` is writable, but it is the only entry under `~`. `~` is also writable, but files created here are not copied over to the host (unlike files created under `~/tmp`.

It seems like a reasonable requirement to make all of `~` readable, but only a subset writable. Using `/tmp/lima` is a workaround, but fragile as files there are automatically deleted after 3 days.