feat(docker-build-push-image): add builder input

[ricky-undeadcoders]

This pull request adds support for specifying a custom Buildx builder in the docker-build-push-image GitHub Action. This enhancement allows users to reuse an existing builder, which is particularly useful for workflows that require persistent caching strategies. The changes update both the documentation and the action implementation to handle the new builder input and ensure the correct builder name is used throughout the workflow.

This is being added to match the functionality that exists in push-to-gar-docker.

Support for custom Buildx builder:

• Added a new builder input to action.yaml and documented its usage in README.md, allowing users to specify a Buildx builder by name. [1] [2]
• Updated the workflow to conditionally set up a new Buildx builder only if a custom builder name is not provided, improving resource reuse and flexibility.
• Introduced a Resolve builder name step to select the appropriate builder name, ensuring that either the user-specified or newly created builder is used consistently.
• Modified the invocation of docker/build-push-action to use the resolved builder name, guaranteeing the correct builder is utilized for image builds.