GitHub action (#1421)

* Implement a re-usable GitHub Action

Implement a GitHub action that can be reused across projects that want
to run black as part of their CI workflows.

* Fix typo in README.md

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

* Use latest Python 3

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

Dockerfile

@@ -0,0 +1,8 @@
+FROM python:3
+
+ENV PYTHONDONTWRITEBYTECODE 1
+ENV PYTHONUNBUFFERED 1
+
+RUN pip install --upgrade --no-cache-dir black
+
+ENTRYPOINT /usr/local/bin/black --check --diff  .

README.md

@@ -36,6 +36,7 @@ _Contents:_ **[Installation and usage](#installation-and-usage)** |
 **[pyproject.toml](#pyprojecttoml)** | **[Editor integration](#editor-integration)** |
 **[blackd](#blackd)** | **[black-primer](#black-primer)** |
 **[Version control integration](#version-control-integration)** |
+**[GitHub Actions](#github-actions)** |
 **[Ignoring unmodified files](#ignoring-unmodified-files)** | **[Used by](#used-by)** |
 **[Testimonials](#testimonials)** | **[Show your style](#show-your-style)** |
 **[Contributing](#contributing-to-black)** | **[Change log](#change-log)** |
@@ -381,6 +382,24 @@ If you're already using Python 3.7, switch the `language_version` accordingly. F
 `stable` is a branch that tracks the latest release on PyPI. If you'd rather run on
 master, this is also an option.
 
+## GitHub Actions
+
+Create a file named `.github/workflows/black.yml` inside your repository with:
+
+```yaml
+name: Lint
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+      - uses: psf/black@stable
+```
+
 ## Ignoring unmodified files
 
 _Black_ remembers files it has already formatted, unless the `--diff` flag is used or

action.yml

@@ -0,0 +1,9 @@
+name: "Black"
+description: "The uncompromising Python code formatter."
+author: "Łukasz Langa and contributors to Black"
+branding:
+  color: "black"
+  icon: "check-circle"
+runs:
+  using: "docker"
+  image: "Dockerfile"