Added Prettier Action

Dockerfile

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+FROM node:latest
+COPY entrypoint.sh ./entrypoint.sh
+COPY package*.json ./
+RUN npm install
+ENTRYPOINT ["./entrypoint.sh"]

action.yml

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+name: 'Prettier Action'
+description: 'Automatically runs prettier on all your pull requests.'
+
+author: Conrad Großer <grosserconrad@gmail.com>
+
+inputs:
+  commit_message:
+    description: Commit message
+    required: false
+    default: 'Prettified Code!'
+  commit_options:
+    description: Commit options
+    required: false
+  file_pattern:
+    description: File pattern used for "git add"
+    required: false
+    default: '*'
+  prettier_options:
+    description: Options for the 'prettier' command
+    required: true
+  branch:
+    description: Branch which the changes are merged to, should be ${{ github.head_ref }} for PRs
+    required: true
+
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
+  args:
+    - ${{ inputs.commit_message }}
+    - ${{ inputs.commit_options }}
+    - ${{ inputs.file_pattern }}
+    - ${{ inputs.prettier_options }}
+    - ${{ inputs.branch }}
+
+branding:
+  icon: 'award'
+  color: 'green'

entrypoint.sh

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+#!/bin/sh -l
+
+set -eu
+
+# Function for setting up git env in the docker container (copied from https://github.com/stefanzweifel/git-auto-commit-action/blob/master/entrypoint.sh)
+git_setup ( ) {
+  cat <<- EOF > $HOME/.netrc
+        machine github.com
+        login $GITHUB_ACTOR
+        password $GITHUB_TOKEN
+        machine api.github.com
+        login $GITHUB_ACTOR
+        password $GITHUB_TOKEN
+EOF
+    chmod 600 $HOME/.netrc
+
+    git config --global user.email "actions@github.com"
+    git config --global user.name "GitHub Actions"
+}
+
+echo "Prettifing files..."
+prettier $INPUT_PRETTIER_OPTIONS
+
+if ! git diff --quiet
+then
+  echo "Commiting and pushing changes..."
+  # Calling method to configure the git environemnt
+  git_setup
+  # Switch to the actual branch
+  git checkout $INPUT_BRANCH
+  # Add changes to git
+  git add "${INPUT_FILE_PATTERN}"
+  # Commit and push changes back
+  git commit -m "$INPUT_COMMIT_MESSAGE" --author="$GITHUB_ACTOR <$GITHUB_ACTOR@users.noreply.github.com>" ${INPUT_COMMIT_OPTIONS:+"$INPUT_COMMIT_OPTIONS"}
+  git push --set-upstream origin "HEAD:$INPUT_BRANCH"
+  echo "Changes pushed successfully."
+else
+  echo "Nothing to commit. Exiting."
+fi