cache doctor rst

Author: smoench

.github/workflows/ci.yaml

@@ -52,7 +52,13 @@ jobs:
             -   name: "Checkout"
                 uses: actions/checkout@v2
 
+            -   name: "Cache DOCtor-RST"
+                uses: actions/cache@v1
+                with:
+                    path: .doctor-rst.cache
+                    key: ${{ runner.os }}-doctor-rst
+
             -   name: "Run DOCtor-RST"
                 uses: docker://oskarstark/doctor-rst
                 with:
-                    args: --short
+                    args: --short --cache-file=/github/workspace/.doctor-rst.cache