Build error for Ubuntu 20.04 Docker container

Building a Docker image for the latest OpenVDS version using the Ubuntu Dockerfile results in the following compilation errors:

/usr/bin/ld: tools/VDSCopy/CMakeFiles/VDSCopy.dir/VDSCopy.cpp.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5' /usr/bin/ld: /lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line

I also tried a manual build resulting in the same error (during make -j8).