Ensure root filesystem is always read-only inside sandbox

Whitelist mode's implicit bwrap root was a writable tmpfs, letting the
sandboxed process create files and directories anywhere not covered by
an explicit ro mount. This was not an issue in blacklist mode due to
--ro-bind / / covering that case.

This patch adds --remount-ro / before any other mount to make the base
layer read-only in both modes.

src/sandbox.rs

@@ -34,6 +34,7 @@ pub fn build_command(config: &SandboxConfig) -> Result<Command, SandboxError> {
         add_ro_bind(&mut cmd, path)?;
     }
 
+    cmd.args(["--remount-ro", "/"]);
     cmd.arg("--new-session");
     cmd.arg("--die-with-parent");
     cmd.arg("--chdir").arg(&config.chdir);