fst/config: allocate filesystem slice

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

fst/config.go

@@ -141,6 +141,12 @@ func (s *SandboxConfig) Bwrap(os linux.System) (*bwrap.Config, error) {
 		Hostname: s.Hostname,
 		Clearenv: true,
 		SetEnv:   s.Env,
+
+		/* this is only 4 KiB of memory on a 64-bit system,
+		permissive defaults on NixOS results in around 100 entries
+		so this capacity should eliminate copies for most setups */
+		Filesystem: make([]bwrap.FSBuilder, 0, 256),
+
 		NewSession:    !s.NoNewSession,
 		DieWithParent: true,
 		AsInit:        true,