A few weeks ago Justin Garrison tweeted:

When I try to do something new I’ve been following the same pattern for a decade

1. Do it in the GUI at least once
2. Do it manually in the CLI
3. Automate it with a script or declarative tool like Terraform
4. (If needed) build a declarative API for others to use

This roughly echoes my own approach to automation, but I've been noticing recently that sometimes I'm going straight to step 3 without even trying 1 & 2.

An example would be multi architecture builds for Docker images. We've been doing x86_64 builds for a while, but we also wanted to do 32bit and 64bit Arm for Raspberry Pis, Apple M1, AWS Graviton etc.

We were already using a GitHub Action to create the x86_64 image, and to get that working from the boilerplate below needed just a couple of things:

1. Create the secrets for DOCKERHUB_USERNAME and DOCKERHUB_TOKEN
2. Fill in the appropriate accountname, imagename and tagname for the tags: line

name: Docker

on:
  workflow_dispatch:

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      -
        name: Set up QEMU
        uses: docker/setup-qemu-action@v1
      -
        name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v1
      -
        name: Login to DockerHub
        uses: docker/login-action@v1 
        with:
          username: ${{ secrets.DOCKERHUB_USERNAME }}
          password: ${{ secrets.DOCKERHUB_TOKEN }}
      -
        name: Build and push
        id: docker_build
        uses: docker/build-push-action@v2
        with:
          push: true
          tags: accountname/imagename:tagname
      -
        name: Image digest
        run: echo ${{ steps.docker_build.outputs.digest }}

To get the Action to make Arm images too all I needed to add was a platforms section after tags:

          tags: |
            accountname/imagename:tagname
            accountname/imagename:latest
            accountname/imagename:GHA${{ github.run_number }}
          platforms: |
            linux/amd64
            linux/arm/v7
            linux/arm64/v8

You'll see that I also added some extra stuff to the tags there to get :latest and a tag associated with the Action run number.

And that's all that was needed. Without ever trying to create a multi-arch image manually, I pressed the run button and it made me images for all 3 architectures. I can't actually imagine how doing it manually would be any easier - even for one iteration never mind for a repeatable system.

NB: I'm using on: workflow_dispatch: here whilst I test the Action, but soon enough it will be plumbed into the pipeline and running without human touch.

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