This action has been updated from the first beta version of GH Actions to the new beta version of GH Actions. See below for the new structure required in your workflow.

A GitHub Action for Custom Jekyll Builds on GitHub Pages

A GitHub Action for building and deploying a Jekyll repo back to its gh-pages branch.

Why not just let GitHub Pages build it? Because this way we can use our own custom Jekyll plugins and build scripts.

Secrets

GITHUB_TOKEN: An access key which scoped to the repository - we need this to push the built site files back to the repo. This is generated for you on each workflow run——see usage in example. (GitHub Provides)

Environment Variables

GITHUB_ACTOR: Username of repo owner or object initiating the action (GitHub Provides)
GITHUB_REPO: Owner/Repository (GitHub Provides)

Example

name: Jekyll Deploy

on: [push]

jobs: 
  build_and_deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v1
      - name: Build & Deploy to GitHub Pages
        env: 
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          GITHUB_REPOSITORY: ${{ secrets.GITHUB_REPOSITORY }}
          GITHUB_ACTOR: ${{ secrets.GITHUB_ACTOR }}
        uses: BryanSchuetz/jekyll-deploy-gh-pages@master

Clones the repo, builds the site, and commits it back to the gh-pages branch of the repository. That's it. Just add the above example to a main.yml file in the .github/workflows directory of your repository—see caveats below.

Caveats

destination: should be set to ./build in your _config.yml file—as God demands.
A Gemfile is required. This must include jekyll and any other custom gems.