Support using container entrypoints for shell tasks

[tclose]

Types of changes

• New feature (non-breaking change which adds functionality)

Summary

When executing shell commands in Docker/Singularity containers, permit setting executable to None in order to use the entrypoint of the container.

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov]

Codecov Report

❌ Patch coverage is 16.07143% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.64%. Comparing base (47c3958) to head (b913f30).
⚠️ Report is 109 commits behind head on main.

Files with missing lines	Patch %	Lines
pydra/environments/tests/test_environments.py	13.04%	20 Missing ⚠️
pydra/environments/base.py	21.42%	11 Missing ⚠️
pydra/compose/shell/builder.py	12.50%	7 Missing ⚠️
pydra/compose/shell/task.py	14.28%	6 Missing ⚠️
pydra/environments/docker.py	0.00%	1 Missing ⚠️
pydra/environments/native.py	0.00%	1 Missing ⚠️
pydra/environments/singularity.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (47c3958) and HEAD (b913f30). Click for more details.

HEAD has 16 uploads less than BASE

Flag	BASE (47c3958)	HEAD (b913f30)
	22	6

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #822       +/-   ##
===========================================
- Coverage   87.89%   57.64%   -30.26%     
===========================================
  Files          84       84               
  Lines       17890    17907       +17     
  Branches     3483     1556     -1927     
===========================================
- Hits        15725    10323     -5402     
- Misses       1809     7399     +5590     
+ Partials      356      185      -171     

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