git context support #1470

Job	Run time
bake (default)	19s
error-msg	8s
bake (release)	20s
docker-config-malformed	8s
error-check	7s
git-context	11s
group	13s
proxy-buildkitd	16s
provenance	12s
provenance (mode=max)	10s
proxy-docker-config	24s
provenance (builder-id=foo)	9s
provenance (false)	21s
provenance (true)	17s
sbom (image, type=image,name=localhost:5000/name/app:latest,push=true)	24s
sbom (binary, /tmp/bake-build)	20s
set	24s
source	1m 6s
standalone	41s
	6m 10s

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