Add pybind11 to dockerfile for system test

[Fujikawas]

According to #696 (comment), the micro-manager participant in two-scale-heat-conduction case depends on pybind11.

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[IshaanDesai]

pybind11 is technically a dependency of micro-dumux, and not of the adapter. Could we install it right before the participant micro-dumux is compiled?

[Fujikawas]

pybind11 is technically a dependency of micro-dumux, and not of the adapter. Could we install it right before the participant micro-dumux is compiled?

You are right. As pybind11 is also not an independent component that we would test etc. like the other adapters/MM, I'd tend to merge it into the installation of Micro Manager since it's often used between C++ case and MM. Also I would like to base the dumux-adapter on preCICE instead of MM.

[IshaanDesai]

You are right. As pybind11 is also not an independent component that we would test etc. like the other adapters/MM, I'd tend to merge it into the installation of Micro Manager since it's often used between C++ case and MM. Also I would like to base the dumux-adapter on preCICE instead of MM.

Makes sense! Let's trigger the system test from this PR once the changes are done.