Always pull, even if image available locally

[JamieMagee]

If we're using a mutable tag, like latest, we should always try and pull the container image regardless of whether or not it's available locally.

https://github.com/dependabot/cli/blob/35deb52c81b16ce706ec8601b85c589b7d483503/internal/infra/run.go#L470-L473

[jakecoffman]

My workflow is I will run script/build which builds the latest image locally and then I can run dependabot, or make changes and run script/dependabot from there on out. I just need to build that initial image because I'm on an ARM machine and we don't currently build multiple architectures.

I'm open to changing the default behavior for a better user experience for users if we can also set it up to be a good experience for core developers too!

[JamieMagee]

Let me open a draft PR to show you what I'm thinking.

[JamieMagee]

See #272