Convert DockerHub webhook to Slack notification

Many Docker registries provide a way to notify team in chat channels when new images are pushed (if you are waiting for a build complete). Let’s add this capability to the official DockerHub registry!

Prerequisites:

• Webhook Relay account
• DockerHub account
• Docker

1. Create a bucket here https://my.webhookrelay.com/buckets
2. Once you have it, in the inputs section you will find your public input endpoint, copy it:

3. Add a new DockerHub webhook setting pointing at our public input endpoint (DockerHub docs):

Push a new Docker image:

$ docker push karolisr/demo-webhook:latest
The push refers to repository [docker.io/karolisr/demo-webhook]
48bd38e03c42: Mounted from karolisr/webhook-demo
fd9f9fbd5947: Mounted from karolisr/webhook-demo
5216338b40a7: Mounted from karolisr/webhook-demo
latest: digest: sha256:703f2bab2ce8df0c5ec4e45e26718954b09bf4a625ab831c6556fd27d60f1325 size: 949

We should be able to see a new incoming webhook. It looks like this:

{
    "push_data": {
        "pushed_at": 1582839308,
        "images": [],
        "tag": "latest",
        "pusher": "karolisr"
    },
    "callback_url": "https://registry.hub.docker.com/u/karolisr/demo-webhook/hook/242ii4ddc2jji4a0cc44fbcdbcdecj1ab/",
    "repository": {
        "status": "Active",
        "description": "",
        "is_trusted": false,
        "full_description": "",
        "repo_url": "https://hub.docker.com/r/karolisr/demo-webhook",
        "owner": "karolisr",
        "is_official": false,
        "is_private": true,
        "name": "demo-webhook",
        "namespace": "karolisr",
        "star_count": 0,
        "comment_count": 0,
        "date_created": 1524557040,
        "repo_name": "karolisr/demo-webhook"
    }
}

Go to the Functions page and click on a “Create Function” button. Enter a name, for example “dockerhub-to-slack” and click “Submit”.

You can now copy/paste webhook payload into the “request body” area for later tests. In the code editor let’s add a function to get repository name and prepare a Slack webhook payload (currently functions have to be written in Lua but more examples for WebAssembly will be added soon):

local json = require("json")

local body, err = json.decode(r.RequestBody)
if err then error(err) end

local message = "New image pushed at: " .. body["repository"]["repo_name"] .. ":" .. body["push_data"]["tag"]

-- Preparing Slack payload
local slack = {
    response_type= "in_channel", 
    text= message}

local result, err = json.encode(slack)

-- Set request header to application/json
r:SetRequestHeader("Content-Type", "application/json")
-- Set request method to PUT
r:SetRequestMethod("POST")
-- Set modified request body
r:SetRequestBody(result)

Click “Save” and then try testing it with the “Send” button:

1. Navigate to https://api.slack.com/messaging/webhooks and click “Create your Slack app”. Select your workspace, enter a name that you will remember.
2. Create a new incoming webhook configuration, copy “Webhook URL” (it starts with https://hooks.slack.com/services/T3...), we will need to supply it to Webhook Relay.
3. Open your bucket details (via https://my.webhookrelay.com/buckets)
4. Open “OUTPUT DESTINATIONS” tab and create a new output called “Slack” with the Slack URL from step 2:

5. Once created, click on the “code” symbol and from the dropdown select dockerhub_to_slack function:

Push new image to DockerHub, you should see a new notification in your Slack channel:

That’s it, feel free to continue modifying Lua function to include pusher’s name and message format. Following this process you can transform any webhook into any other webhook.

Have fun!