v0.14: Dockerfile: Add g++-multilib to docker image

This commit adds the `g++-multilib` package to the docker image,
which will fix build errors for some 32-bit platforms if we use
this docker image to build test cases on 64-bit machines.

Fixes: https://github.com/zephyrproject-rtos/docker-image/issues/48

Signed-off-by: Henry Wang <Henry.Wang@arm.com>