Tweak Docker storage to handle healthcheck issues

[joshmeek]

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 self.flows.values() instead of keys() since the flow path was moved to a value. Pip installs from the current master branch of Prefect until a new version is released which contains all new storage and environment changes

Why is this PR important?

Avoids issues like:

Step 9/9 : RUN python /root/.prefect/healthcheck.py
 ---> Running in 203394a563b9
Beginning health check...
Traceback (most recent call last):
  File "/root/.prefect/healthcheck.py", line 5, in <module>
    with open(flow_file, 'rb') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'Test-Flow'

and

Step 9/9 : RUN python /root/.prefect/healthcheck.py
 ---> Running in 6b18ac805330
Beginning health check...
Traceback (most recent call last):
  File "/root/.prefect/healthcheck.py", line 6, in <module>
    flow = cloudpickle.load(f)
ModuleNotFoundError: No module named 'prefect.environments.execution'

[codecov]

Codecov Report

Merging #968 into master will not change coverage.
The diff coverage is n/a.