[docker] Separate cache volumes for builds against protected and unprotected

What does this MR do?

Isolate cache volumes between protected and unprotected branches

Why was this MR needed?

To have better separation between caches for protected and uprotected local caches.

What's the best way to test this MR?

Run builds from the same project, once setting the branch to protected and unprotected. See, that 2 different volumes are used to cache the project locally.

What are the relevant issue numbers?

closes: gitlab#547332 (closed)