Accept vanilla Linux Foundation signoffs

This allows signing off commits with `git commit -s`

Not sure if this is personal preference (you're welcome to ignore it), but I would use `?` instead of `|`+blank (especially `| *`)
```
dcoPrefix='(Docker-DCO-1.1-)?Signed-off-by:'
dcoRegex="^$dcoPrefix ([^<]+) <([^<>@]+@[^<>]+)>( \\(github: ($githubUsernameRegex)\\))?$"
```