Comment # 1 on bug 1215465 from Felix Niederwanger

Looks like a issue with man itself to me:

 $ man -P cat docker-build > 123
 troff:<standard input>:80: warning: cannot select font 'C'
 ...

 $ man docker-build > 123
 troff:<standard input>:80: warning: cannot select font 'C'
 ...