hst/container: optional runtime and tmpdir sharing

Sharing and persisting these directories do not always make sense. Make it optional here.

Closes #16.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/sandbox/case/mapuid.nix

@@ -36,6 +36,8 @@ in
   userns = false;
   x11 = false;
   hostAbstract = false;
+  shareRuntime = true;
+  shareTmpdir = true;
 
   # 0, PresetStrict
   expectedFilter = {