docker: Add missing package g++-multilib
The Ubuntu 18 image already has this, but 16 and 20 are missing this package. It is required for cross compiling some 32 bit packages (e. g. qtwebengine) on 64 bit hosts.
The Ubuntu 18 image already has this, but 16 and 20 are missing this package. It is required for cross compiling some 32 bit packages (e. g. qtwebengine) on 64 bit hosts.