Allow to add custom ignores to .dockerignore

My current `.dockerignore` file is vast. Possibly with a couple of relicts, but I want to carry over parts of it.
I don't expect dockerfile-rails to expose every minor edge case as a config option.

I would like to have the chance to provide my own ignores.

```dockerignore
/.adr-dir
/.bundle/
/.devcontainer/
/.dockerignore
/.dpl
/.editorconfig
.env*
!.env.example
!.env.test
/.git*
/.gitlab-ci.yml
/.gitlab/
/.idea/
.overmind.sock
.pnp.js
/.pnp/
/.rspec
/.sass-cache
/.slugignore
/.vscode/
/.yarn/
/**/.DS_Store
/**/.gitignore
/app.json
/Aptfile
/bin/setup
CHANGELOG.md
/charts/
/config/fly.*.toml
/coverage/
/doc/
!/doc/api/*/swagger.yaml
/docker-compose.*.yml
/docker-compose.yml
/Dockerfile
/fly.toml
/log/
/node_modules/
/Procfile
/Procfile.dev
/public/
!/public/robots.txt
!/public/.well-known/
/terraform/
/tmp/
/yarn-error.log
```

## What are popular ignores you may find in my `.dockerignore`-file?

- VS Code and Devcontainer
- IaC (helm, terraform)
- CI/CD (.gitlab and .gitlab-ci.yml)

## Prior art

This has been requested in the past as an alternative approach for #87.