CI: run all steps in a pandoc container

Author: tarleb

.github/workflows/ci.yaml

@@ -32,13 +32,15 @@ jobs:
             # This should be the oldest version that's supported
             # - 2.19.2
 
+    container:
+      image: pandoc/core:${{ matrix.pandoc }}
+
     steps:
       - name: Checkout
         uses: actions/checkout@v2
 
+      - name: Install dependencies
+        run: apk add make
+
       - name: Test
-        run: |
-          # Use docker image with current release.
-          make test \
-            PANDOC="docker run --rm --volume $(pwd):/data \
-              --user $(id -u):$(id -g) pandoc/core:${{ matrix.pandoc }}"
+        run: make test