Use GHA caching built into Docker Buildx

[GaryGSC]

The Docker actions can now natively use the GitHub Actions cache, though things haven't quite gotten through the release pipeline.

This should work as-is, but once things have been released, we'll no longer need to explicitly use v0.6.0 of Buildx.

Using docker/build-push-action@v2 in CI allows us to get rid of

with:
  install: true

If someone using this template were to build multiple Docker images, they'd want to specify a scope to avoid cache collision/invalidation.

[github-actions]

Terraform Plan:

will create 1 resource:

• local_file - appspec_json

will update 2 resources:

• aws_s3_bucket - my_s3_bucket_logs
• aws_lambda_function - test_lambda

will replace (delete then create) 1 resource:

• aws_ecs_task_definition - task_def

see details

[GaryGSC]

This repo has a pretty small Dockerfile, so the effects aren't very pronounced, but here are the stats:

• CI before, which wasn't using any caching - 9s to set up Buildx, 23s to build
• Deploy before - 7s to set up Buildx, 4s to manually pull down cache, 15s to build, 8s to upload cache
• CI with new workflow, which populated the cache - 8s to set up Buildx, 26s to build
• CI re-run, which used the cache - 11s to set up Buildx, 3s to build

Supposedly, the time to set up Buildx should go down once a little once it's natively available in the GitHub runner, but it seems basically identical already.