move building process to ./buildTmp directory in repository

[tlwt]

that way the local build preserves data.

add ./buildTmp to .gitignore

[tlwt]

must be done in docker-compose for local builds:

     volumes:
        - ./:/drone/custombuild
        - ./buildTmp:/data

but dont forget to update .gitignore

buildTmp/*
!buildTmp/README.md