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

Attention: Patch coverage is 16.07143% with 47 lines in your changes missing coverage. Please review.

Project coverage is 64.11%. Comparing base (058d793) to head (38a1db8).

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 (058d793) and HEAD (38a1db8). Click for more details.

HEAD has 16 uploads less than BASE

Flag	BASE (058d793)	HEAD (38a1db8)
	23	7

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #822       +/-   ##
===========================================
- Coverage   88.76%   64.11%   -24.66%     
===========================================
  Files          84       84               
  Lines       17890    17907       +17     
  Branches     3483     1556     -1927     
===========================================
- Hits        15880    11481     -4399     
- Misses       1640     6024     +4384     
- Partials      370      402       +32     

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