refactor: always rebuild dev-generic

The `dev-generic` job was originally optimized to only build once per workflow run.  Subsequent re-runs skipped the build steps.  This unfortunately made it difficult to analyze dev-generic cache problems after merging.  This change removes this optimization.

The cache key has a timestamp suffix so that the cache is always built anew.  This provides more integrity, traceability and flexibility at the cost of increased cache turnover across the repository.

PR: zmkfirmware/zmk-docker#74

.github/workflows/containers.yml

@@ -88,6 +88,7 @@ jobs:
           echo ::set-output name=major-minor-branch::${MAJOR_MINOR_BRANCH}
   dev-generic:
     needs:
+      - timestamp
       - tags
     if: ${{ !startsWith(github.ref, 'refs/tags') }}
     runs-on: ubuntu-latest
@@ -133,18 +134,14 @@ jobs:
           cache-name: dev-generic
         with:
           path: ${{ steps.paths.outputs.local }}
-          key: ${{ runner.os }}/${{ env.cache-name }}/${{ github.run_id }}
-      - name: Rebuild cache?
-        id: should-rebuild
-        run: echo ::set-output name=value::${{ steps.cache.outputs.cache-hit != 'true' || env.no-cache == 'true' }}
+          key: ${{ runner.os }}/${{ env.cache-name }}/${{ github.run_id }}/${{ needs.timestamp.outputs.timestamp }}
+          restore-keys: |
+            ${{ runner.os }}/${{ env.cache-name }}/${{ github.run_id }}
       - name: Set up QEMU
-        if: ${{ steps.should-rebuild.outputs.value == 'true' }}
         uses: docker/setup-qemu-action@v1
       - name: Set up Docker Buildx
-        if: ${{ steps.should-rebuild.outputs.value == 'true' }}
         uses: docker/setup-buildx-action@v1
       - name: Build to local cache
-        if: ${{ steps.should-rebuild.outputs.value == 'true' }}
         uses: docker/build-push-action@v2
         with:
           target: dev-generic
@@ -152,6 +149,7 @@ jobs:
             ZEPHYR_VERSION=${{ env.zephyr-version }}
           no-cache: ${{ env.no-cache == 'true' }}
           cache-from: |
+            type=local,src=${{ steps.paths.outputs.local }}
             type=registry,ref=${{ steps.paths.outputs.branch }}
             ${{ (steps.paths.outputs.base != '') && format('type=registry,ref={0}', steps.paths.outputs.base) || '' }}
             type=registry,ref=${{ steps.paths.outputs.major-minor-branch }}
@@ -159,7 +157,7 @@ jobs:
             type=registry,ref=${{ steps.paths.outputs.major-minor-branch-upstream }}
           cache-to: type=local,dest=${{ steps.paths.outputs.local-new }},mode=max
       - name: Push to registry cache
-        if: ${{ (steps.should-rebuild.outputs.value == 'true') && (env.docker-hub-credentials == 'true') }}
+        if: ${{ env.docker-hub-credentials == 'true' }}
         uses: docker/build-push-action@v2
         with:
           target: dev-generic
@@ -174,12 +172,12 @@ jobs:
       # https://github.com/docker/build-push-action/issues/252
       # https://github.com/moby/buildkit/issues/1896
       - name: Switch local cache
-        if: ${{ steps.should-rebuild.outputs.value == 'true' }}
         run: |
           rm -rf ${{ steps.paths.outputs.local }}
           mv ${{ steps.paths.outputs.local-new }} ${{ steps.paths.outputs.local }}
   candidates:
     needs:
+      - timestamp
       - architectures
       - tags
       - dev-generic
@@ -286,7 +284,7 @@ jobs:
           cache-name: dev-generic
         with:
           path: ${{ steps.paths.outputs.dev-generic }}
-          key: ${{ runner.os }}/${{ env.cache-name }}/${{ github.run_id }}
+          key: ${{ runner.os }}/${{ env.cache-name }}/${{ github.run_id }}/${{ needs.timestamp.outputs.timestamp }}
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v1
       - name: Set up Docker Buildx