Fix issue with user config path being overwritten in docker storage

[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.

[codecov]

Codecov Report

Merging #1719 into master will not change coverage.
The diff coverage is 100%.