bake: forward default ssh agent for remote bake file

[jedevc]

Not quite how, but at some point, ReadRemoteFiles stopped working with git remotes.

$ docker buildx bake "git@github.com:docker/buildx.git" --print binaries --progress=plain
#0 building with "container" instance using docker-container driver

#1 [internal] load git source git@github.com:docker/buildx.git
#1 ERROR: Unimplemented: unknown service moby.sshforward.v1.SSH
------
 > [internal] load git source git@github.com:docker/buildx.git:
------
ERROR: failed to find target binaries

When cloning git remotes, we need to attach the SSH agent service, so that this works:

docker buildx bake "git@github.com:docker/buildx.git" --print binaries --progress=plain
#0 building with "container" instance using docker-container driver

#1 [internal] load git source git@github.com:docker/buildx.git
#1 1.723 ref: refs/heads/master	HEAD
#1 1.849 6535f16aec98d4e94e3a5f6c9dbe736a0f407f7a	HEAD
#1 3.583 6535f16aec98d4e94e3a5f6c9dbe736a0f407f7a	refs/heads/master
#1 DONE 3.7s

#1 [internal] load git source git@github.com:docker/buildx.git
#1 CACHED
{
  "group": {
    "default": {
      "targets": [
        "binaries"
      ]
    }
  },
  "target": {
    "binaries": {
      "context": "git@github.com:docker/buildx.git",
      "dockerfile": "Dockerfile",
      "args": {
        "BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
        "GO_VERSION": "1.20"
      },
      "target": "binaries",
      "platforms": [
        "local"
      ],
      "output": [
        "./bin/build"
      ]
    }
  }
}