Replace deprecated command with environment file

[jongwooo]

Description of your changes

Closes #175

Update .github/workflows/docker-image.yml to use environment file instead of deprecated set-output command. For more information, see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

I found the workflow file that use set-output command through the following command:

$ find . -name '*.yml' -o -name '*.yaml' | xargs egrep '\bset-output\b'

AS-IS

echo "::set-output name=strategy::$strategy"

TO-BE

echo "strategy=$strategy" >> $GITHUB_OUTPUT

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https://dart-review.googlesource.com/c/dart-docker/+/344040 has been updated with the latest commits from this pull request.