ci: Run test on both platforms

.github/workflows/ci.yml

@@ -46,8 +46,14 @@ jobs:
       tag: ${{ env.TAG }}
 
   test:
+    name: test (${{ matrix.platform }})
     needs: build
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        platform:
+          - linux/amd64
+          - linux/arm64
     steps:
 
     - uses: docker/login-action@v1
@@ -60,11 +66,18 @@ jobs:
       with:
         python-version: ~3
 
+    # The ubuntu-latest runner is linux/amd64 so anything else will
+    # run with emulation, which is still better than nothing.
+    - if: matrix.platform != 'linux/amd64'
+      uses: docker/setup-qemu-action@v2
+
     - name: Run zika-tutorial
       run: |
         git clone https://github.com/nextstrain/zika-tutorial
         pip install nextstrain-cli
         nextstrain build --image ghcr.io/nextstrain/base:${{ needs.build.outputs.tag }} zika-tutorial -F
+      env:
+        DOCKER_DEFAULT_PLATFORM: ${{ matrix.platform }}
 
   push-branch:
     if: startsWith(needs.build.outputs.tag, 'branch-') && github.event_name != 'pull_request'