Add .dockerignore file

[haani-niyaz]

The current Dockerfile does a copy all via COPY . . without any filtering on what is in the working directory. We should add a .dockerignore file to eliminate bloat. The current landscape has many things that can be ignored:

.
├── Dockerfile
├── LICENSE
├── Makefile
├── README.md
├── bin
│   └── app
├── cmd
│   └── main.go
├── coverage.out
├── docs
│   ├── continuous-integration
│   │   ├── gitleaks.md
│   │   ├── golangci-lint.md
│   │   ├── goreleaser.md
│   │   ├── img
│   │   │   └── mkdocs-gh-pages.png
│   │   ├── mkdocs-material.md
│   │   ├── pre-commit.md
│   │   └── semantic-release.md
│   ├── index.md
│   └── quickstart.md
├── go.mod
└── mkdocs.yml

Acceptance Criteria

1. Add a .dockerignore file to ignore docs, binaries, test artefacts, github actions etc.