Make base images for protocol builds configurable.

[dmjb]

Allows the base image used in protocol builds to be overridden with thv run --runtime-image

[codecov]

Codecov Report

❌ Patch coverage is 45.58824% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.97%. Comparing base (865272e) to head (b47a20b).

Files with missing lines	Patch %	Lines
pkg/config/interface.go	16.66%	10 Missing ⚠️
pkg/config/config.go	30.76%	9 Missing ⚠️
pkg/container/templates/templates.go	64.70%	5 Missing and 1 partial ⚠️
pkg/runner/protocol.go	58.33%	3 Missing and 2 partials ⚠️
pkg/runner/config_builder.go	0.00%	4 Missing ⚠️
pkg/runner/retriever/retriever.go	0.00%	2 Missing ⚠️
pkg/mcp/server/run_server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3662      +/-   ##
==========================================
- Coverage   65.99%   65.97%   -0.03%     
==========================================
  Files         415      416       +1     
  Lines       41192    41253      +61     
==========================================
+ Hits        27186    27218      +32     
- Misses      11911    11937      +26     
- Partials     2095     2098       +3     

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[github-actions]

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[github-actions]

✅ PR size has been reduced below the XL threshold. The size review has been dismissed and this PR can now proceed with normal review. Thank you for splitting this up!