(core) Add caching to Docker workflows

Summary:
Updates GitHub workflows to cache image builds. This should
speed up most builds.

Test Plan: Tested manually in fork of grist-core.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3363

.github/workflows/docker.yml

@@ -39,3 +39,5 @@ jobs:
           push: true
           platforms: linux/amd64,linux/arm64/v8
           tags: ${{ steps.meta.outputs.tags }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max

.github/workflows/docker_latest.yml

@@ -34,6 +34,7 @@ jobs:
           context: .
           load: true
           tags: ${{ github.repository_owner }}/grist:latest
+          cache-from: type=gha
       - name: Use Node.js ${{ matrix.node-version }} for testing
         uses: actions/setup-node@v1
         with:
@@ -64,6 +65,8 @@ jobs:
           platforms: linux/amd64,linux/arm64/v8
           push: true
           tags: ${{ github.repository_owner }}/grist:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
       - name: Update latest branch
         uses: ad-m/github-push-action@8407731efefc0d8f72af254c74276b7a90be36e1
         with: