Runc should not be writing anything on truly read only container rootfs

[rhvgoyal]

Now people are looking into truly read-only container rootfs where same rootfs will be shared by multiple containers. Right now runc assumes that container rootfs is writable and will create mount point destinations (file/dir) so that mounts can succeed.

I am looking for functionality where caller is responsible to make sure mount destination exist otherwise runc will simply fail the opertion.

Provide a knob for truly read-only container where runc does not try to scribble anything on container rootfs and if something is missing, runc simply errors out.

[rhvgoyal]

cc @mrunalp

[mrunalp]

Created opencontainers/runtime-spec#591 to handle mount destinations.

[cyphar]

We have slowly chipped away at requiring write access to the rootfs for a while (I think at the time this was opened, our pivot_root(2) logic still required a temporary directory be created -- which hasn't been true for a long time now).

At this point, we do still create mountpoint targets (because we need to maintain compatibility) but those will error out so I think that matches the overall goal of this issue despite not being exactly what you wanted.