Docker prefect version

[cicdw]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Uses -U to override base image's prefect version (if present).

cc: @bnaul

[codecov]

Codecov Report

Merging #1461 into master will not change coverage.
The diff coverage is n/a.