This was the indirect cause of the error: there was tmpstderror twice in the shell file, so I couldn't see the output of latexmk which shows in the log (also on Travis, I hope) what exactly went wrong. Now tmpstdout is printed as well.
PS Your explanation of docker was spot on, it was very easy to find this bug now with the correct commands :)
This was the indirect cause of the error: there was
tmpstderrortwice in the shell file, so I couldn't see the output of latexmk which shows in the log (also on Travis, I hope) what exactly went wrong. Nowtmpstdoutis printed as well.PS Your explanation of docker was spot on, it was very easy to find this bug now with the correct commands :)