Add optional `docker_platforms` input to control Docker build architecture(s)

[HDash]

Summary

Add an optional input to specify the target platform(s) for the Docker image build, threaded through to the platforms: field of the docker/build-push-action@v6 step. The default would be empty, so the action omits platforms: entirely and builds for the runner's native architecture exactly as it does today — no behaviour change for existing users.

Motivation

Currently the Docker build (action.yml, the "🐳🚀 Build and Push to Docker Registry" step) sets no platforms: field, so every image is built only for the GitHub runner's native arch (linux/amd64). Users who want multi-arch images — e.g. linux/arm64 for Apple Silicon / ARM servers — have no way to request them without forking the action.

Proposed change

1. New action input (action.yml), default empty:

   docker_platforms:
     description: >
       Comma-separated platforms to build the Docker image for, e.g.
       'linux/amd64,linux/arm64'. Leave empty (default) to build only for the
       runner's native architecture.
     default: ''

2. Conditionally pass it to the build step (action.yml:704-721):

   platforms: ${{ inputs.docker_platforms }}

   An empty string here is treated by build-push-action as "no platforms set", so the default reproduces current behaviour.
3. Thread the parameter through the R generators so it lands in user workflows:
   • R/use_workflow.R (add docker_platforms alongside the other docker_* args)
   • R/fill_yaml.R (add to the with2/arg list)
   • inst/templates/rworkflows_template.yml with: block
   • keep .github/workflows/rworkflows_static.yml in sync

Notes / caveats

• Cross-building linux/arm64 on an amd64 runner requires docker/setup-qemu-action@v3 before the build step. We should add it (it's a no-op when only the native platform is built).
• Multi-platform images can't be loaded into the local daemon, only pushed — fine here since the step already uses push: true.
• cache-from/cache-to: type=gha remain compatible with multi-platform builds.

Backward compatibility

Fully backward compatible: empty default ⇒ no platforms: ⇒ identical to the current single-arch build.