Fix docker layer caching in build workflow

* Only cache docker layers on Linux
* Add target OS name to cache key (futureproofing)

.github/workflows/build.yml

@@ -16,6 +16,10 @@ jobs:
 
       - name: Cache Docker layers
         uses: satackey/[email protected]
+        with:
+          key: docker-layers-linux-{hash}
+          restore-keys: |
+            docker-layers-linux-
         # Ignore the failure of a step and avoid terminating the job.
         continue-on-error: true
 
@@ -38,11 +42,6 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v2
 
-      - name: Cache Docker layers
-        uses: satackey/[email protected]
-        # Ignore the failure of a step and avoid terminating the job.
-        continue-on-error: true
-
       - name: Build Docker builder image
         run: docker build --build-arg UID=$(id -u) --build-arg GID=$(id -g) -f .github/docker/windows/Dockerfile -t builder .