Fix DCO parsing for numeric github handles like cpuguy83

Turns out, "alphanumeric" actually means both "alpha" AND "numeric". Dur.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6883 from tianon/fix-dco-parsing-for-numeric

Fix DCO parsing for numeric github handles like cpuguy83