"$@" works differently on ash and dash. `foo="$@"` 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