fix: frontend/gateway: leave mount type as nil for Windows

[billywr]

Added a check to exclude Windows OS when adding bind mount types to container image bundle.
Bind types apply to Linux OS; hcsshim accepts empty bind types for Windows container creation.
#4892

More info, what I was trying to achieve

1. I attempted to build a custom Docker frontend image(resides in dockerhub:docker.io/100909/dockerfile-4892) using the following Dockerfile:

   FROM mcr.microsoft.com/windows/nanoserver:ltsc2022 AS release
   LABEL moby.buildkit.frontend.network.none="true"
   LABEL moby.buildkit.frontend.caps="moby.buildkit.frontend.inputs,moby.buildkit.frontend.subrequests,moby.buildkit.frontend.contexts"
   COPY /dockerfile-frontend.exe /bin/dockerfile-frontend.exe
   ENTRYPOINT ["/bin/dockerfile-frontend.exe"]
   
   FROM release

   The dockerfile-frontend.exe was built from buildkit/frontend/dockerfile/cmd/dockerfile-frontend.

2. I created another Dockerfile referencing the custom frontend image:

   # syntax=docker.io/100909/dockerfile-4892

3. I ran the following command:

   buildctl build --frontend=dockerfile.v0 --local context=path_to_dockerfile_in_step_2 --local dockerfile=path_to_dockerfile_in_step_2 --output type=image,name=testimage,push=false

   This resulted in an error during container creation in hcsshim:

   error: failed to solve: failed to create shim task: invalid OCI spec - Type 'bind' not supported: unknown
   

4. I made changes in BuildKit to fix the error in step 3 and ran buildctl again.

   Container creation stage passes,

I am getting a new error shown below when building the custom dockerfile frontend as in step 2.
sh error: failed to solve: exit code: 2

This commit fixes the error in step 3.
@tonistiigi I am seeking ideas and comments.

[xtrm-en]

Don't forget to sign-off your commits, see this failed run.

[thompson-shaun]

Is this PR close to a ready state @billywr? Just wanted to make sure it didn't get lost ;)