It turned out that some sources where bind-mounted read-write
which should have been read-only:
* Extensions
* Fonts
* resolv.conf
* localtime
* machine-id
Many of these are not generally writable by the user, but
some are, which is pretty bad. For instance if the an
app is user-installed it will be able to write to its
locale data.
We fix this by using --ro-bind where needed.