default to git context

[crazy-max]

follow-up https://github.com/docker/bake-action/pull/181

[crazy-max]

~I put this one back in draft while we wait for auth support before defaulting: https://github.com/docker/buildx/pull/2363~

[crazy-max]

https://github.com/docker/bake-action/actions/runs/12300037568/job/34336154468?pr=193#step:5:255 is not related

/usr/bin/docker buildx bake --allow fs=* --set *.output=/tmp/bake-build --set *.cache-from=type=gha,scope=attests-binary --set *.cache-to=type=gha,scope=attests-binary,mode=max --metadata-file /home/runner/work/_temp/docker-actions-toolkit-si9yJo/bake-metadata-3093f93292.json --provenance mode=max,builder-id=https://github.com/docker/bake-action/actions/runs/12300037568/attempts/3 --sbom true binary
#0 building with "builder-f2c9d3e0-f8b5-4a52-b04c-877d7b196b0a" instance using docker-container driver

#1 [internal] load local bake definitions
#1 reading docker-bake.hcl 242B / 242B done
#1 DONE 0.0s
ERROR: failed to evaluate path "/tmp/bake-build": lstat /tmp/bake-build: no such file or directory

Being fixed in https://github.com/docker/buildx/pull/2860

[tonistiigi]

Make sure to highlight this in changelog once we cut the next version.