Fix issue where ACTS was built with unresolved symbols in the library.

- Updated underlying container to Debian/testing-20210111 with gcc 10.2.1
- Moved buildcache convenience script to upstream container
- added old ACTS package.py file (probably not important and can be removed for the next version)
- rolled back cmake to 3.18.1 (not important, can be changed again for next version)
- rolled back Eigen to 3.3.7 (again, not important, should be upped again in next release)