Ability to run a shell command

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**Tell us about your request**
What do you want us to build?

A shell command option to run a command/terminal against your container live without manually changing a build command (like docker exec). A way to, on the fly, run shell command e.g. for database migrations or running management commands.

**Describe alternatives you've considered**
- Spinning up another instance on EC2/ECS to connect to my RDS instance to run migrations/revert
- Attaching a build script that uses environment variables and just re-deploying the app after changing a variable

This originally came to me when wondering how i'd actually revert a database migration on a production deployment with App Runner, it isnt really possible by the looks of it at the moment; unless I run a build command which isn't interactive or fast.

The issue names no implementation files, tests, or entry points; begin by reviewing the App Runner container workflow and the Docker exec alternative described here. Define how a command or interactive terminal would run against a live deployment for migrations and management commands, with completion meaning this works without changing and redeploying a build command.

Written by the indexing model from the issue text.