docker: install g++-multilib

[PolynomialDivision]

Node fails to compile with:

  In file included from /usr/include/c++/8/memory:62,
  from ../deps/v8/src/libplatform/default-foreground-task-runner.h:8,
  from ../deps/v8/src/libplatform/default-foreground-task-runner.cc:5:
  /usr/include/c++/8/bits/stl_algobase.h:59:10: fatal error: bits/c++config.h:
    No such file or directory
  #include <bits/c++config.h>
          ^~~~~~~~~~~~~~~~~~
  compilation terminated.

Add g++-multilib to fix this.

Fixes: https://github.com/openwrt/packages/issues/17074

Signed-off-by: Nick Hainke vincent@systemli.org

[aparcar]

Merged, thank you!