Add `dockerfile` input parameter

[som-matrix]

Problem

The action currently supports image input for pre-built images, but lacks a dockerfile input for specifying custom Dockerfile paths when building from source.

Proposed Solution

Add a dockerfile input that gets passed to flyctl deploy --dockerfile.

Usage

- name: Deploy PR app to Fly.io
  uses: superfly/fly-pr-review-apps@1.2.1
  with:
    dockerfile: ./docker/Dockerfile

Implementation

In entrypoint.sh around line 57, add dockerfile parameter to the deploy command:

flyctl deploy --dockerfile "$dockerfile" --config "$config" --app "$app" ...

This would complement the existing image input and help users with non-standard Dockerfile locations.