Fix defects from PR#777

[jmgrady]

PR #777 introduced 2 defects:

• in certmgr/Dockerfile, the destination directory for the entrypoint scripts needs a trailing '/'
• from the review, the python scripts were updated to use pathlib instead of strings for file paths. One function, Path.readlink() is not available until Python 3.9; the certmgr container is at Python 3.7. The 2 uses of Path.readlink() are changed to use os.readlink()

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This change is 

[codecov-io]

Codecov Report

Merging #781 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #781   +/-   ##
=======================================
  Coverage   51.44%   51.44%           
=======================================
  Files         235      235           
  Lines        6276     6276           
  Branches      404      404           
=======================================
  Hits         3229     3229           
  Misses       2749     2749           
  Partials      298      298           

Flag	Coverage Δ
#backend	55.83% <ø> (ø)
#frontend	47.32% <ø> (ø)

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[imnasnainaec]

Reviewed 3 of 3 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jmgrady)