feat: add variable `commit_date` #6936

Job	Run time
bake	17s
custom-labels-annotations	10s
docker-push	56s
flavor	10s
global-exps	8s
images	10s
json	10s
multi-images	14s
sep-tags ( )	7s
sep-tags (,)	14s
tag-match (\d.\d.\d, 0)	11s
tag-match (\d.\d, 0)	14s
tag-schedule	7s
tag-semver (auto)	10s
tag-match (v(.*), 1)	13s
bake-annotations	21s
tag-schedule (cron-{{date 'YYYYMMDD'}})	8s
tag-semver (true)	12s
tag-schedule ({{date 'YYYYMMDD-HHmmss'}})	10s
tag-semver (false)	6s
tag-schedule (schedule)	6s
bake-cwd	19s
no-images	7s
output-env	16s
sha-short	6s
sha-short (16)	12s
	5m 34s

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