feat: create Dockerfile & automatically build on release

[kelly-sovacool]

Changes

• Create a Dockerfile with the package installed.
• Create a GitHub Action that automatically builds and pushes the container to dockerhub when a new release is cut on GitHub.

Issues

Resolves #9

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• [ ] Write unit tests for any new features, bug fixes, or other code changes.
• [ ] Update the docs if there are any API changes (roxygen comments, vignettes, readme, etc.).
• [ ] Update NEWS.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/
• Run devtools::check() locally and fix all notes, warnings, and errors.