Add support for GitHub Actions ci workflow on PRs

[victorpopkov]

Problem

Similar to dstmodders/docker-imagemagick#23:

Currently, CI workflow is not enabled for PRs. It would be nice to have that enabled for both main and develop branches.

Proposed solution

Adding this to .github/workflows/ci.yml should be enough:

on:
  pull_request:
    branches:
      - develop
      - main

Progress

• Include on.pull_request.branches with develop and main within .github/workflows/ci.yml.