f20 - update docker requirements and specify 2 vs 3

BUG=None
TEST=./build-dockerimage

Change-Id: I88729cfc31bbfdcdec63830e4b395aa8063d554a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3237601
Auto-Submit: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Jesse McGuire <jessemcguire@google.com>
Reviewed-by: Seewai Fu <seewaifu@google.com>
Tested-by: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
5 files changed