use ADD instead of COPY for `.git`

[stephanos]

What was changed

Why?

https://github.com/temporalio/temporal/actions/runs/8299631664/job/22717105934?pr=5507

server.Dockerfile:27
--------------------
  25 |     # Git info is needed for Go build to attach VCS information properly.
  26 |     # See the `buildvcs` Go flag: https://pkg.go.dev/cmd/go
  27 | >>> COPY ./.git ./.git
  28 |     COPY ./.gitmodules ./.gitmodules
  29 |     RUN --mount=type=cache,target=/root/.cache/go-build (cd ./temporal && make temporal-server)
--------------------
ERROR: failed to solve: source can't be a git ref for COPY
Error: buildx bake failed with: ERROR: failed to solve: source can't be a git ref for COPY

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?