fix: resolve Docker build pipeline failures

[hal-bot]

Summary

Fixes a series of failures in the CI/CD Docker build pipeline.

Changes

• backend/.dockerignore → backend/Dockerfile.dockerignore — Renamed so BuildKit picks it up when the build context is the repo root (not the backend/ subdirectory). Updated exclusion paths to be repo-root-relative.
• backend/Dockerfile — Widened COPY paths to work from repo root context; added parent pom.xml copy so Maven can resolve the relativePath; added --mount=type=cache for /root/.m2 to speed up repeated builds; added checkstyle.xml to the COPY step so the Checkstyle plugin can find it.
• .github/workflows/ci-cd.yml — Updated docker-build job to use context: . and file: backend/Dockerfile; removed now-redundant artifact download steps.
• docker-compose.yml — Aligned local build config with CI (context: ., dockerfile: backend/Dockerfile).
• .husky/pre-commit & .husky/pre-push — Added nvm sourcing so npx/npm are found when committing/pushing via VS Code's Source Control UI.