feat(docker-build-push-multiarch): allow github hosted runner usage

[ricky-undeadcoders]

This pull request updates the configuration for the multi-architecture Docker build workflow to provide more flexible and explicit control over runner types. The previous server-size input has been replaced by new runner type inputs, allowing users to specify instance types for x64, arm64, and manifest builds, with sensible defaults based on the chosen runner environment (self-hosted or github). Several variable names have been standardized, and job definitions have been updated to use the new runner type outputs.

Runner configuration enhancements:

• Added new workflow inputs: runner-type, runner-type-x64, runner-type-arm64, and runner-type-manifest to allow explicit control over the instance types used for each build architecture. The runner-type input sets the default environment, with overrides available for each architecture. [1] [2]
• Implemented a new step in the prepare-matrix job to determine and export the correct runner types for x64, arm64, and manifest builds, using either user-provided values or sensible defaults based on the runner-type input.

Variable and output standardization:

• Renamed output variables and matrix keys from runner_arches to runner-arches for consistency. [1] [2]

Job runner updates:

• Updated prepare-matrix, build-and-push, and merge-digest jobs to use the new runner type outputs, ensuring that each job runs on the appropriate instance type for its architecture. [1] [2] [3]

A successful run of this can be seen here: https://github.com/grafana/wait-for-github/actions/runs/18846461222/job/53771699421?pr=386