If I understand it correctly, the default propagation type is private,
but f**king systemd mounts everything as shared. This is especially
problematic when doing recursive bind, which is not our case, but still
better to explicitly set propagation to private anyway.

See: https://lwn.net/Articles/689856/, https://lwn.net/Articles/690679/

Resolves #3

Resolves #3

See https://github.com/alpinelinux/aports/pull/3886

"$@" works differently on ash and dash. `foo="$@"` assigns only the
first item (according to IFS) to the variable on dash.

It's useless for NBD image and probably causes the following error:

    > Formatting image to ext4
    mke2fs 1.43.4 (31-Jan-2017)
    Discarding device blocks:    4096/1310720               failed - I/O error