ci: Enable multiplatform builds for amd64 and arm64

[matthewfeickert]

Description

Add the platforms argument to the docker/build-push-action GitHub Action and pass the platform names to enable build for both amd64 and arm64 architectures. This will enable Apple M1 and M2 machines to run the pyhf/pyhf Docker images without emulation which will significantly improve runtime performance.

While this hasn't been run through the CI, this has been tested locally with

$ docker buildx build --file docker/Dockerfile --platform linux/arm64 --tag pyhf/pyhf:debug-arm64 --load .
$ docker image inspect pyhf/pyhf:debug-arm64 | jq '.[] | with_entries(select(.key == "Architecture"))'
{
  "Architecture": "arm64"
}

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add the 'platforms' argument to the docker/build-push-action GitHub
  Action and pass the platform names to enable build for both amd64 and
  arm64 architectures. This will enable Apple M1 and M2 machines to run
  the pyhf/pyhf Docker images without emulation which will significantly
  improve runtime performance.

[codecov]

Codecov Report

Base: 98.30% // Head: 98.30% // No change to project coverage 👍

Coverage data is based on head (48122b6) compared to base (57a5ad7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2058   +/-   ##
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  Coverage   98.30%   98.30%           
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  Files          69       69           
  Lines        4530     4530           
  Branches      645      645           
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  Hits         4453     4453           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	97.88% <ø> (ø)
doctest	61.14% <ø> (ø)
unittests-3.10	96.31% <ø> (ø)
unittests-3.8	96.33% <ø> (ø)
unittests-3.9	96.35% <ø> (ø)

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