Adds a Dockerfile for local development

[danielhartnell]

λ ~/ docker build -t linkhack .
λ ~/ docker run -p 3000:3000 linkhack
=> Booting Puma
=> Rails 6.1.7.7 application starting in development
=> Run `bin/rails server --help` for more startup options
Puma starting in single mode...
* Puma version: 6.1.0 (ruby 3.2.1-p31) ("The Way Up")
*  Min threads: 5
*  Max threads: 5
*  Environment: development
*          PID: 1
* Listening on http://0.0.0.0:3000
Use Ctrl-C to stop

It already uses SQLite so no need to introduce docker compose to bring up supporting resources like Postgres.

[khaleksa]

LGTM! Thank you for updating ruby version