Create a Docker Action

In this tutorial, you’ll build a custom Docker Action that:

1. Triggers when a pull request is opened.
2. Fetches a random “thank you” GIF from Giphy.
3. Posts a comment with the GIF on the PR using the GitHub REST API.

This pattern can be adapted to any third-party API integration.

External APIs

We rely on two REST APIs:

Giphy API: Random GIF Endpoint

Request URL:

https://api.giphy.com/v1/gifs/random?api_key=YOUR_GIPHY_API_KEY&tag=thank%20you&rating=g

Sample response:

{
  "data": {
    "type": "gif",
    "id": "l119IDMNbVskgyf5u",
    "images": {
      "original": { "url": "https://media1.giphy.com/media/l119IDMNbVskgyf5u/giphy.gif" },
      "downsized": { "url": "https://media1.giphy.com/media/l119IDMNbVskgyf5u/200.gif" }
    },
    "title": "sci-fi comedy GIF by Ghosted"
  }
}

GitHub REST API: Post a Comment

Use the Issues API to add a comment:

curl -s -X POST \
  -H "Accept: application/vnd.github.v3+json" \
  -H "Authorization: Bearer $GITHUB_TOKEN" \
  -d '{"body":"![GIF](GIF_URL)\nThank you for this contribution!"}' \
  https://api.github.com/repos/OWNER/REPO/issues/ISSUE_NUMBER/comments

1. Create the Repository

1. On GitHub, create a new public repo named docker-action-pr-giphy-comment.
2. Initialize with a README.md.

After creation, you’ll see the default commit:

Add Giphy API Key as a Secret

1. Go to Settings → Secrets and variables → Actions.
2. Add a new secret GIPHY_API_KEY with your Giphy API key.

Once saved, the secret appears in the list:

2. Define the Action Structure

In your repo root, create:

• Dockerfile
• entrypoint.sh
• action.yml

Dockerfile

FROM alpine:3.10

# Install curl and jq
RUN apk update && apk add --no-cache curl jq

# Copy and set entrypoint
COPY entrypoint.sh /entrypoint.sh
RUN chmod +x /entrypoint.sh

ENTRYPOINT ["/entrypoint.sh"]

Entrypoint Script (entrypoint.sh)

This script retrieves a GIF and posts a comment on the PR:

#!/bin/sh
set -e

GITHUB_TOKEN=$1
GIPHY_API_KEY=$2

# Get PR number
pr_number=$(jq --raw-output .pull_request.number "$GITHUB_EVENT_PATH")
echo "PR #$pr_number"

# Fetch GIF
response=$(curl -s "https://api.giphy.com/v1/gifs/random?api_key=$GIPHY_API_KEY&tag=thank%20you&rating=g")
gif_url=$(echo "$response" | jq --raw-output .data.images.downsized.url)
echo "GIF URL: $gif_url"

# Post comment
comment=$(curl -s -X POST \
  -H "Authorization: token $GITHUB_TOKEN" \
  -H "Accept: application/vnd.github.v3+json" \
  -d "{\"body\":\"### PR #$pr_number\n\n🎉 Thank you for this contribution!\n\n![GIF]($gif_url)\"}" \
  "https://api.github.com/repos/$GITHUB_REPOSITORY/issues/$pr_number/comments")

echo "Comment posted: $(echo "$comment" | jq --raw-output .html_url)"

Make it executable:

chmod +x entrypoint.sh

Action Metadata (action.yml)

name: "Giphy PR Comment"
description: "Add a Giphy GIF comment to new pull requests."
inputs:
  github-token:
    description: "GitHub token for API calls"
    required: true
  giphy-api-key:
    description: "Giphy API key"
    required: true
runs:
  using: "docker"
  image: "Dockerfile"
  args:
    - ${{ inputs.github-token }}
    - ${{ inputs.giphy-api-key }}

3. Test Your Action

Create a test workflow at .github/workflows/test.yml:

on:
  pull_request:
    types: [opened]

jobs:
  testing-action:
    runs-on: ubuntu-latest
    permissions:
      issues: write
      pull-requests: write
    steps:
      - name: Checkout code
        uses: actions/checkout@v3

      - name: Post PR comment
        uses: ./
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          giphy-api-key: ${{ secrets.GIPHY_API_KEY }}

Create the workflow folder:

mkdir -p .github/workflows

4. Commit, Push, and Trigger

1. Commit all changes and push to GitHub.
2. Edit README.md on a new branch and open a pull request.

When the PR opens, your action runs automatically:

You’ll see a comment from the github-actions bot on your pull request:

Next Steps

You’ve built and tested a Docker-based GitHub Action that posts Giphy comments on PRs. To take it further:

• Publish your action to the GitHub Marketplace.
• Consume the published action in other repositories.

Links and References

• Giphy API Documentation
• GitHub REST API: Issues
• GitHub Actions Guide