TEST_MAXFILESIZE should be usable together with USESAFEDOCKER

[ifrh]

In README.md it is mentioned:

The maximum size of a file produced by a user submission (setting TEST_MAXFILESIZE, currently not supported with USESAFEDOCKER, until http://stackoverflow.com/questions/25789425 is resolved)

In http://stackoverflow.com/questions/25789425, which has been posted in Nov 2014, there exists an answer from the year 2015 with a pointer to moby/moby#4717 which has been closed. ( cf. moby/moby#4717 (comment) )

[ifrh]

seems to be implementet in PR #354

https://github.com/KITPraktomatTeam/Praktomat/pull/354/files#diff-990f53fe06b9e396f4b76a0a746410e78d99eb7653a07c887a3f529b0381f0a6R91-R93

after merging that PR only removing from README.md the lines is needed:

 * The maximum size of a file produced by a user submission (setting
   `TEST_MAXFILESIZE`, currently not supported with `USESAFEDOCKER`, until
http://stackoverflow.com/questions/25789425 is resolved)