Failure retrieving commit time with SOURCE_DATE_EPOCH set (`fork/exec /usr/bin/git: no such file or directory`)

Contributing guidelines and issue reporting guide

• I've read the contributing guidelines and wholeheartedly agree. I've also read the issue reporting guide.

Well-formed report checklist

• I have found a bug that the documentation does not mention anything about my problem
• I have found a bug that there are no open or closed issues that are related to my problem
• I have provided version/information about my environment and done my best to provide a reproducer

Description of bug

Bug description

When using git context with a subdir (<repo>.git#<ref>:<subdir>) and SOURCE_DATE_EPOCH is set, the build fails to get commit time for : git stderr: : fork/exec /usr/bin/git: no such file or directory.

ERROR: failed to build: failed to solve: failed to read dockerfile: failed to get commit time for https://github.com/moby/buildkit.git: git stderr:
: fork/exec /usr/bin/git: no such file or directory

Expected behaviour - no failure, buildkit is using passed SOURCE_DATE_EPOCH value.

Reproduction

docker buildx create --name repro --driver docker-container --use

# SOURCE_DATE_EPOCH set, failure
SOURCE_DATE_EPOCH=1700000000 docker buildx build --call check \
  "https://github.com/moby/buildkit.git#master:examples/create-certs"

# SOURCE_DATE_EPOCH unset, success
docker buildx build --call check \
  "https://github.com/moby/buildkit.git#master:examples/create-certs"

Version information

% docker --version
Docker version 29.8.0, build 88096ef

% docker buildx version       
github.com/docker/buildx v0.37.0 ac30b249211430b85fb8f37b6e7154b5c47ba0b6

Reference

• #3565

Reproduce the failure with the provided docker buildx build command using a git context with a subdir and SOURCE_DATE_EPOCH set, then inspect the git-context handling for that combination. Done means the build completes without the fork/exec /usr/bin/git error and uses the passed SOURCE_DATE_EPOCH value.