Ability to save the Dockerfile created by `build_docker_image` deployment step

[discdiver]

First check

• [X] I added a descriptive title to this issue.
• [X] I used the GitHub search to find a similar request and didn't find it.
• [X] I searched the Prefect documentation for this feature.

Prefect Version

2.x

Describe the current behavior

No Dockerfile is persisted.

Describe the proposed behavior

A Dockerfile is persisted.

Example Use

The user could then inspect the Dockerfile to make deployments with Docker more transparent.

The user could also then modify the Dockerfile and specify the modified Dockerfile in prefect.yaml.

Additional context

I talked to @desertaxle about this a bit already.

In my own use and at PACC it was observed that this functionality would be helpful for debugging.

[serinamarie]

I think if we removed deletion of the Dockerfile at the end of prefect_docker.steps.build_docker_image, the Dockerfile will persist, but the build step will still have dockerfile: auto which would no longer be accurate the next time that the build step was run, as we'd want to use the Dockerfile they have. So we'd want to think about that.