ci: add PR checks and harden outputs

[AdekunleBamz]

This PR improves reliability and contributor experience for transaction-latency by adding PR-time checks, tightening Docker build context, and hardening runtime behavior.

Why

• There was no CI coverage on pull requests (only a Docker push workflow on main), so regressions could merge unnoticed.
• Docker builds could accidentally include sensitive/local files (like .env or data outputs) in the build context.
• The tool could fail writing CSVs if the output directory didn’t exist, and receipt polling treated all RPC errors the same.

Changes

• Add a new CI workflow that runs on PRs and main pushes:
  • gofmt check
  • go vet ./...
  • go test ./...
  • Docker build (no push)
• Add .dockerignore to exclude .env, data/, git metadata, etc.
• Improve runtime robustness:
  • Validate TO_ADDRESS as a proper hex address and disallow zero address.
  • Create output directory before writing CSV.
  • Only sleep on receipt-not-found; fail fast on other receipt errors.
• Expand README with clearer setup/run steps and output file locations.

Verification

• Open the PR and confirm the new CI workflow runs on GitHub Actions.
• (Optional) Build/run locally:
  • docker build -t transaction-latency .
  • docker run --rm -it -v "$(pwd)/data:/app/data" --env-file .env transaction-latency