.github: cross-compile docker images

Build docker images for amd64, arm64, and arm/v7.  Also several other
small improvements to docker workflow like documenting which actions we
run on, setting concurrency settings, and reversing the logic for when
we push new images live (specifically looking for a push event rather
than a "not pull request" event, in case we add other events later).

Fixes #26

Author: willnorris

.github/workflows/docker.yml

@@ -2,15 +2,22 @@ name: Docker
 
 on:
   push:
+    # push events will publish a new image, so only trigger on main branch or semver tags.
     branches: [ 'main' ]
     tags: [ 'v*' ]
   pull_request:
+    # Run the workflow on pull_request events to ensure we can still build the image.
+    # We only publish the image on push events (see if statements in steps below).
     branches: [ 'main' ]
 
 env:
   REGISTRY: ghcr.io
   IMAGE_NAME: ${{ github.repository }}
 
+concurrency:
+  group: ${{ github.workflow }}-$${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   build-and-push-image:
     runs-on: ubuntu-latest
@@ -27,7 +34,7 @@ jobs:
 
     - name: Log into registry ${{ env.REGISTRY }}
       uses: docker/login-action@f4ef78c080cd8ba55a85445d5b36e214a81df20a # v2.1.0
-      if: github.event_name != 'pull_request'
+      if: github.event_name == 'push'
       with:
         registry: ${{ env.REGISTRY }}
         username: ${{ github.actor }}
@@ -44,16 +51,17 @@ jobs:
       uses: docker/build-push-action@c56af957549030174b10d6867f20e78cfd7debc5 # v3.2.0
       with:
         context: .
-        push: ${{ github.event_name != 'pull_request' }}
+        push: ${{ github.event_name == 'push' }}
         tags: ${{ steps.meta.outputs.tags }}
         labels: ${{ steps.meta.outputs.labels }}
+        platforms: linux/amd64,linux/arm64,linux/arm/v7
 
     # Sign the Docker image
     - name: Install cosign
-      if: github.event_name != 'pull_request'
+      if: github.event_name == 'push'
       uses: sigstore/cosign-installer@9becc617647dfa20ae7b1151972e9b3a2c338a2b #v2.8.1
     - name: Sign the published Docker image
-      if: github.event_name != 'pull_request'
+      if: github.event_name == 'push'
       env:
         COSIGN_EXPERIMENTAL: "true"
       run: cosign sign ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build-and-push.outputs.digest }}