scheduler: add docker info to host.serialnum

[davidpanderson]

This includes version and type,
and also whether WSL and/or docker use are banned by prefs. From this we can find the number of Docker-capable hosts.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 11.94%. Comparing base (d5bfd9a) to head (47056f5).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
sched/handle_request.cpp	0.00%	29 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6179      +/-   ##
============================================
- Coverage     11.95%   11.94%   -0.01%     
  Complexity     1068     1068              
============================================
  Files           278      278              
  Lines         36942    36963      +21     
  Branches       8531     8538       +7     
============================================
  Hits           4416     4416              
- Misses        32125    32146      +21     
  Partials        401      401              

Files with missing lines	Coverage Δ
sched/handle_request.cpp	0.00% <0.00%> (ø)

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