Allow specifying CmdRedirect as user command

[Keno]

If specified they will override the global stdio config. A primary motivation is to pass non-stdio fds, cf
JuliaLang/julia@79a5ccd.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.14%. Comparing base (f9ef4ec) to head (dc88518).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Docker.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
- Coverage   75.37%   75.14%   -0.23%     
==========================================
  Files           6        6              
  Lines         666      676      +10     
==========================================
+ Hits          502      508       +6     
- Misses        164      168       +4     

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