Automatic merge from submit-queue (batch tested with PRs 38277, 36361, 38452)
Add options for build container rsync optimization
KUBE_RSYNC_COMPRESS env var sets rsync compression level.
KUBE_RSYNC_GENERATED_TO_BUILD_CONTAINER env var disables rsyncing
generated files to build containers.
Why KUBE_RSYNC_COMPRESS is needed -- from rsync manual on `--compress` option (implied by non-zero `--compress-level`):
> Note that this option typically achieves better compression ratios than can be achieved by using a compressing remote shell or a compressing transport because it takes advantage of the implicit information in the matching data blocks that are not explicitly sent over the connection.
Use case for `KUBE_RSYNC_GENERATED_TO_BUILD_CONTAINER`: when you sometimes build stuff locally (e.g. `make WHAT=cmd/kubectl`) and sometimes do it on remote docker (`build-tools/run.sh make WHAT=cmd/hyperkube`), local builds touch generated files which causes them to be rsynced to the build data container, which may slow down the builds. Still, I'm not sure whether local->remote rsync of generated files is useful (e.g. someone may want to edit generated files for debugging purposes?), so I made not rsyncing these files an option instead of forcing such behavior.
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