Add build step for non-linear deal.II example

[MakisH]

We currently only test for the linear example of deal.II:

systemtests/adapters/Dockerfile.dealii-adapter

Lines 14 to 19 in eb8ca76

	RUN git clone --branch $branch https://github.com/precice/dealii-adapter.git
	RUN cd dealii-adapter/linear_elasticity/ && \
	cmake -DDEAL_II_DIR=$HOME/dealii -DDIM=3 -DCMAKE_BUILD_TYPE=Release . && \
	make -j $(nproc)
	WORKDIR /home/precice/dealii-adapter

It would be very helpful to at least build also the non-linear version. This way we could catch issues such as precice/dealii-adapter#30.

Adding a complete system test would also be nice.