Change how the build directory is chosen

Description

Whilst doing !876 (closed) we found some issues on how the runner chooses the build directory, which were explain it detail in !876 (comment 93721659)

Proposal

• Remove RootDir
• The responsability of choosing the build directory should only be Build#StartBuild to prevent the dual brain problem
• disallowedCustomBuildDir should be moved to Build#StartBuild as well after the Docker#prepareBuildsDir is done for mounting the volumes for the docker executre.

Links to related issues and merge requests / references

!876 (comment 93721659)

#2211 (closed)

!876 (closed)