Use a hash of the docker folder as a key to GHA cache (#1001)

This should be the only thing impacting docker cache.

Author: mayeut

.github/workflows/build.yml

@@ -51,15 +51,11 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
 
-      - name: Create cache timestamp
-        id: timestamp
-        run: echo "::set-output name=timestamp::$(python3 -c 'from datetime import datetime,timezone; print(datetime.now(timezone.utc).isoformat())')"
-
       - name: Setup cache
         uses: actions/cache@v2
         with:
           path: .buildx-cache-${{ matrix.policy }}_${{ matrix.platform }}/*
-          key: buildx-cache-${{ matrix.policy }}-${{ matrix.platform }}-${{ steps.timestamp.outputs.timestamp }}
+          key: buildx-cache-${{ matrix.policy }}-${{ matrix.platform }}-${{ hashFiles('docker/**') }}
           restore-keys: buildx-cache-${{ matrix.policy }}-${{ matrix.platform }}-
 
       - name: Build