Add docker_buildx_name param

[jpdsteno]

Checklist

• [x] All new jobs, commands, executors, parameters have descriptions
• [x] Examples have been added for any significant new features
• [x] README has been updated, if necessary

Motivation, issues

https://github.com/CircleCI-Public/aws-ecr-orb/issues/364

Description

This PR adds a new optional field named docker_buildx_name which allows the caller the specify an override for the default Docker buildx instance name. This should avoid races and collisions when concurrently uploading the DLC cache across build jobs.