cft: optimize dockerfile for caching

BUG=b:228645277
TEST=./build-dockerimage.sh, and manual testing

Change-Id: I5cebdc1751aca2ba42172d2f2e2e63635508ea6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3579929
Tested-by: Derek Beckett <dbeckett@chromium.org>
Auto-Submit: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Jaques Clapauch <jaquesc@google.com>
Commit-Queue: Jaques Clapauch <jaquesc@google.com>
1 file changed