feat(gentoo): update outdated Gentoo Docker images

This PR addresses issue #41 (closed). Gentoo Docker images are outdated (more than 20+ days old as of today) and it is necessary to run a full system update prior to executing salt-boostrap, which takes very long and eventually fails. A workaround is to temporarily switch to the ksmanis/stage3 image. Once Gentoo Docker images are published again, this PR should be reverted.