ch-docker2tar misformats file size and name status line

On my Debian 9 test box, ch-docker2tar misformats the file size and name:

$ ch-docker2tar mpihello-mpich $CH_TEST_TARDIR < /dev/null
reidpr 24

Expected output is:

108M [...]/mpihello-mpich.tar.gz

The problem is on line 36 of ch-docker2tar, which even bears a comment that it's brittle. It used to work.