Honor nested .dockerignore exclusions in Docker build context

[cristianrgreco]

Summary of changes

• precompute build-context entries and apply @balena/dockerignore filtering before tarring
• ensure the selected Dockerfile is always present in tar entries
• add regression fixture and integration test for nested exclusion patterns (* with !subdir/nested)

Verification commands run

• npm run format
• npm run lint
• npm test -- packages/testcontainers/src/generic-container/generic-container.test.ts -t "dockerignore"

Test results summary

• both dockerignore integration tests pass:
  • should honour .dockerignore file
  • should honour nested .dockerignore exclusion patterns

Closes #775

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