build: Ephemeral environments for PRs via slash command (#13189)

* First pass at ephemeral env, new Docker ci target

* Add service checks, get public IP

* Separate issue_comment and workflow_run jobs

* Refactor workflows

* Adjust comment author association

* Checkout code

* Fix image name, manage service desired task count

* Use merge commit sha

* Fix IP output, add failure comment

* Refactor comment parsing & env spinup

* Check container image publish status

* Parse AWS account ID from registry URL

* Use PR number rather than variable merge commit SHA for image tag

* Fix docker push conditional

* Push multiple tags to ECR

* Fix comment author check

* Refactor comment body check

* Provision service with active task to get correct IP

* /testenv up

* Add @mentions to PR comments, env var cleanup