Build fails when shared context directory contains non-ASCII characters

[maxproske]

Initially caught by docker/compose#13965 2 Compose services sharing a build context containing Chinese characters, but after some digging, every build dialed over gRPC fails. Same as #2793 which was closed as not reproducible.

Sanitize the shared key before it reaches gRPC.

Steps to reproduce:

mkdir -p 早

docker buildx create --name repro --driver docker-container
docker buildx build --builder repro 早

docker buildx bake -f - a b <<'EOF'
target "a" { context = "早" }
target "b" { context = "早" }
EOF

docker compose -f - build <<'EOF'
services:
  a: {build: 早}
  b: {build: 早}
EOF

All 3 fail with:

failed to dial gRPC: rpc error: code = Internal desc = rpc error: code = Internal desc = header key "x-docker-expose-session-sharedkey" contains value with non-printable ASCII characters

[maxproske]

Newbie question: if approved, worth me guarding downstream in buildkit too?