Allow NFS Export in Container Filesystem with overlay2

[rdlarkins]

Description
NFS Export is disabled in the container filesystem when the storage driver is overlay2. The option is set by docker when mounting the filesystem as at least in the past, there were issues with enabling the 'index' option, this was changed in this PR.

In a bit of testing I've done, it seems like at least the immediate issues previously triggered by enabling 'index' no longer occur with recent docker/ubuntu (Docker version 20.10.12, build e91ed57 running on Ubuntu 20.04.4 LTS with kernel 5.13.0-30-generic).

Using aufs as the storage driver does allow for this behavior, but as aufs support is deprecated in docker it would be better to support this with the main storage driver. A suggestion in the comments of the PR linked would be to add an option to enable both 'index' and 'nfs_export' from docker only in the case where you want export enabled for a container.

[rdlarkins]

Code where index option is set to off:

moby/daemon/graphdriver/overlay2/overlay.go

Line 574 in d809ad9

if readonly {

[sungaok]

@rdlarkins
hi,
can these code work to run or test right now?
I want use this function right now.

[CobraFlow]

Hi,
I have a requirement for nfs_export=on and index=on inside a docker container. I am building a PXE boot server.
I have a mounted volume(ext4) containing a 'virgin' root filesystem. I create a RW folder for each connected client using the virgin root as the lower and the upper for the client's persistence area.

...Host
mnt /dev/sda /mnt/storage


...Container
mkdir -p /mnt/storage/test/someos
mkdir -p /mnt/storage/test/upperX
mkdir -p /mnt/storage/test/workX
mkdir -p /mnt/storage/test/userX

mount  -t overlay overlay -o nfs_export=on,index=on \
    -o lowerdir=/mnt/storage/test/someos \
    -o upperdir=/mnt/storage/test/upperX \
    -o workdir=/mnt/storage/test/workX \
    /mnt/storage/test/userX

I then want to NFS export the resulting /mnt/storage/test/userX

[thaJeztah]

@neersighted @dmcgowan could you have a peek?

[CobraFlow]

Has anyone 'peeked' at this yet? @neersighted @dmcgowan ... 🙏

[CobraFlow]

Has anyone had a look at this yet? I'm going to need a solution soon as I'm a bit stuck without this...🤔😳🤞