Fix 21h1 hcsshim::CreateComputeSystem error, push images and run integration tests in MRs

What does this MR do?

1. Fix 21h1 hcsshim::CreateComputeSystem during integration tests for 21h1
2. Update the way the helper image is computed on the docker executor so that the ongoing revision is used instead

Why was this MR needed?

Unbreak main branch

What's the best way to test this MR?

No failure in windows 21H1 integration tests 3/6

What are the relevant issue numbers?

close #29037 (closed)