build-yocto/Dockerfile: Install package `tree`
build-yocto/Dockerfile: Install package `tree`

Fix https://github.com/gmacario/easy-jenkins/issues/79

Signed-off-by: Gianpaolo Macario <d50c09b51e61901a1c5054dce86f92776c198faa@mentor.com>
gmacario/easy-build