Last night myself and a colleague both tried clean builds with LANG=pl_PL.UTF-8 in order to try and narrow down the cause of the failure, but our builds were successful (we were both building master, I built Odroid_C2, my colleague built RPi2).
In my case I built with the following patch on my Ubuntu 17.10 build system (which uses en_GB.UTF-8 by default): 12K6
Would you mind testing the following patch on your Docker container to see if it has any effect? 13e9
You should restore your system to the initial failure condition, ie. pl_PL.UTF-8 as the system default LANG, before the test.
This isn't necessarily a long term fix (although if it works then it could be), but it might also help shed more light on the issue.
Can you also confirm your command line when starting your build, and if you are building with any other patches?