Docker build improvements

See merge request memri/pod!187

Nobody used this API at this moment
because API wasn't working until 2 commits ago.

While not clean to change without deprecation warnings and all,
it should be safe to do so now.

Also, move validation code to the new proper file

In Rust, binary crates aren't meant to be IT-tested:
https://doc.rust-lang.org/book/ch11-03-test-organization.html#integration-tests-for-binary-crates

https://users.rust-lang.org/t/integration-tests-for-binary-crates/21373

This change aligns us better with Rust-s best practices.
Additionally, there won't be duplication
of run unit tests anymore.