When switching from dockerized make to normal make, `make clean` fails.

$ make clean  
Removing auxiliary files...
rm: cannot remove './tmp/main.nav': Permission denied
rm: cannot remove './tmp/main.out': Permission denied
rm: cannot remove './tmp/main.fdb_latexmk': Permission denied
rm: cannot remove './tmp/main.bcf': Permission denied
rm: cannot remove './tmp/main.upa': Permission denied
rm: cannot remove './tmp/main.run.xml': Permission denied
rm: cannot remove './tmp/main.log': Permission denied
rm: cannot remove './tmp/main.vrb': Permission denied
rm: cannot remove './tmp/main.snm': Permission denied
rm: cannot remove './tmp/main.xdv': Permission denied
rm: cannot remove './tmp/main.aux': Permission denied
rm: cannot remove './tmp/main.toc': Permission denied
make: *** [Makefile:81: clean] Error 1

These files are only accessible by root. We should at least document how to correct that.