ci(build): build Docker images on pull requests without pushing

Because we've been having issues with these builds across platforms, start building images (but not
pushing) on PRs to make sure things don't break before merging in

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,5 +6,4 @@ If you're adding a new image, make sure you have done the following.
 
 * [ ] Added to lint workflow matrix (`.github/workflows/lint.yml`)
 * [ ] Added to build workflow matrix (`.github/workflows/build.yml`)
-* [ ] Added to build workflow matrix (`.github/workflows/build-pr.yml`)
 * [ ] Added to Makefile (`Makefile`)

.github/workflows/build.yml

@@ -6,6 +6,7 @@ on:
       - main
   schedule: # Run everyday
     - cron: "0 0 * * *"
+  pull_request:
 
 jobs:
   build:
@@ -30,7 +31,7 @@ jobs:
       - uses: docker/build-push-action@v3
         with:
           context: ${{ matrix.elixir }}
-          push: true
+          push: ${{ github.event_name != 'pull_request' }}
           tags: articulate/articulate-elixir:${{ matrix.elixir }}
           platforms: linux/amd64,linux/arm64/v8
           cache-from: type=registry,ref=articulate/articulate-elixir:${{ matrix.elixir }}