hst/config: handle filesystem entry targeting root

This allows any fstype supported by hst to be directly mounted on sysroot. A special case in internal/app applies the matching entry early and excludes it from path hiding.

Closes #5.

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

internal/app/seal_linux.go

@@ -244,8 +244,14 @@ func (seal *outcome) finalise(ctx context.Context, sys sys.State, config *hst.Co
 			Tty:          true,
 			AutoEtc:      true,
 
-			AutoRoot:  container.AbsFHSRoot,
-			RootFlags: container.BindWritable,
+			Filesystem: []hst.FilesystemConfigJSON{
+				{&hst.FSBind{
+					Target:   container.AbsFHSRoot,
+					Source:   container.AbsFHSRoot,
+					Write:    true,
+					AutoRoot: true,
+				}},
+			},
 		}
 
 		// bind GPU stuff