builder: support `--parent-bootstrap` for merge

This option allows merging multiple bootstraps of upper layer with
the bootstrap of a parent image, so that we can implement container
commit operation for nydus image.

Signed-off-by: Yan Song <[email protected]>

Merge pull request dragonflyoss#1122 from imeoer/prepare-v2.2-release

dep: prepare for v2.2 release

Merge pull request dragonflyoss#1107 from jiangliu/service-macos-unus…

…ed.patch

service: fix a unused variable warning on macos

Merge pull request dragonflyoss#1095 from jiangliu/v2.1-compat

rafs: reserve bits in RafsSuperFlags to be compatible with v2.2

Merge pull request dragonflyoss#1083 from imeoer/nydusify-fix-prefetch

nydusify: upgrade deps to fix lost prefetch table

Merge pull request dragonflyoss#1016 from imeoer/fix-release-pipeline

release: fix pipeline hang when compile for amd64

Merge pull request dragonflyoss#1017 from jiangliu/v6-underflow-2.1

rafs: fix a underflow bug in rafs v6 implementation

Merge pull request dragonflyoss#940 from changweige/2.1-fix-graceful-…

…exit

nydusd: register signal hanlder earlier

Merge pull request dragonflyoss#839 from imeoer/stable/v2.1-cherry-pick

[backport to stable/v2.1] storage: add mirror health checking support

Merge pull request dragonflyoss#804 from imeoer/bring-auto-version

release: update version on build automatically