This fixes #5804.
This fixes #5804.

The problem was that btrfs send wants a snapshot to be readonly (to prevent
interim modification), but we were not passing the "-r" flag when creating btrfs
subvolume for copying the snapshots of a source container.

Signed-off-by: Free Ekanayaka <04111f73b2d444cf053b50d877d79556bf34f55a@canonical.com>

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