Add Docker/Compose deployment support and publish container images to GHCR

[trcjr]

Summary

This PR adds first-class containerization support for goPool and wires container image publishing into the release workflow.

What changed

• added a multi-stage Dockerfile
  • Go 1.26 build stage
  • ZeroMQ build/runtime dependencies
  • minimal Debian runtime image
• added docker-compose.yml
  • persistent ./data bind mount
  • configurable ports
  • basic healthcheck
• added env.example
  • build metadata
  • port mappings
  • network mode
  • runtime flags
• updated release workflow
  • added build-container job
  • builds multi-arch images for linux/amd64 and linux/arm64
  • pushes to ghcr.io/${{ github.repository }}
  • adds packages: write permission required for GHCR publishing
• updated docs
  • README container quick start
  • documentation index link
  • operations guide container section
• added vendor/ to .gitignore

Why

This makes goPool easier to:

• deploy reproducibly
• run with persistent config/state via ./data
• test locally with Docker or Compose
• publish and consume as a versioned container image

Notes

• the container build preserves the existing CGO + ZeroMQ requirements
• image build args include BUILD_TIME and BUILD_VERSION
• compose is set up for a bind-mounted ./data directory
• GHCR publishing now has the required package permissions

Validation

• local Docker build succeeds
• container starts with mounted ./data
• Compose workflow supports background run/logs/stop flow
• release workflow now includes container image publishing

[Distortions81]

Been meaning to make one for it, thanks for the PR!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.96%. Comparing base (6a501e9) to head (1c2b013).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #28   +/-   ##
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  Coverage   33.96%   33.96%           
=======================================
  Files         131      131           
  Lines       21316    21316           
=======================================
  Hits         7240     7240           
  Misses      12637    12637           
  Partials     1439     1439           

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